Substring instead of Left

One simple and quick solution could be to use an ordinal splitter to divide the string at '_' into rows. Then there's no need to fiddle around with LEFT and SUBSTRING and CHARINDEX.

The ordinal splitter can be found here

CREATE FUNCTION dbo.DelimitedSplit8K
--===== Define I/O parameters
        (@pString VARCHAR(8000), @pDelimiter CHAR(1))
--WARNING!!! DO NOT USE MAX DATA-TYPES HERE!  IT WILL KILL PERFORMANCE!
RETURNS TABLE WITH SCHEMABINDING AS
 RETURN
--===== "Inline" CTE Driven "Tally Table" produces values from 1 up to 10,000...
     -- enough to cover VARCHAR(8000)
  WITH E1(N) AS (
                 SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL
                 SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL
                 SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1
                ),                          --10E+1 or 10 rows
       E2(N) AS (SELECT 1 FROM E1 a, E1 b), --10E+2 or 100 rows
       E4(N) AS (SELECT 1 FROM E2 a, E2 b), --10E+4 or 10,000 rows max
 cteTally(N) AS (--==== This provides the "base" CTE and limits the number of rows right up front
                     -- for both a performance gain and prevention of accidental "overruns"
                 SELECT TOP (ISNULL(DATALENGTH(@pString),0)) ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) FROM E4
                ),
cteStart(N1) AS (--==== This returns N+1 (starting position of each "element" just once for each delimiter)
                 SELECT 1 UNION ALL
                 SELECT t.N+1 FROM cteTally t WHERE SUBSTRING(@pString,t.N,1) = @pDelimiter
                ),
cteLen(N1,L1) AS(--==== Return start and length (for use in substring)
                 SELECT s.N1,
                        ISNULL(NULLIF(CHARINDEX(@pDelimiter,@pString,s.N1),0)-s.N1,8000)
                   FROM cteStart s
                )
--===== Do the actual split. The ISNULL/NULLIF combo handles the length for the final element when no delimiter is found.
 SELECT ItemNumber = ROW_NUMBER() OVER(ORDER BY l.N1),
        Item       = SUBSTRING(@pString, l.N1, l.L1)
   FROM cteLen l
;

Then the query is very simple and executes efficiently

with cte (col) as (
    select 'T101184_7AB_C1_ABCDE' union all
    select 'T1011847ABC1ABCDE')
select Item
from cte c
     cross apply
     (select case when charindex('_',c.col)=0 then 'NA' else c.col end chr) ndx
     cross apply
     dbo.DelimitedSplit8K(ndx.chr, '_')
where ItemNumber=1;

Output

Item
T101184
NA

One way is

with cte (col) as 

(select 'T101184_7AB_C1_ABCDE' union all
 select 'T1011847ABC1ABCDE')

 select coalesce(nullif(substring(col,0,charindex('_',col)),''),'NA')
 from cte;

Having said that, there is no reason you shouldn't use left for the problem you have at hand