Computer program’s more competent, holding confusion back (9)
I believe the answer is:
assembler
'computer program's' is the definition.
(assembler is a kind of computer program)
'more competent holding confusion back' is the wordplay.
'more competent' becomes 'abler' ('able' can be similar in meaning to 'competent').
'holding' means one lot of letters goes inside another.
'confusion' becomes 'mess' (mess can mean confusion or chaos).
'back' says the letters should be written in reverse.
'mess' reversed gives 'ssem'.
'abler' going around 'ssem' is 'ASSEMBLER'.
(Other definitions for assembler that I've seen before include "made to recompile computer program" , "Builder" , "production-line worker" , "Person piecing something together" , "Code-converting computer program" .)