Work done, thus holds back (4)
I believe the answer is:
opus
'work' is the definition.
(I've seen this before)
'done thus holds back' is the wordplay.
'done' becomes 'up' (both can mean finished or at an end).
'thus' becomes 'so' (synonyms).
'holds' indicates putting letters inside.
'back' says the letters should be written in reverse.
'up' put into 'so' is 'supo'.
'supo' backwards is 'OPUS'.
(Other definitions for opus that I've seen before include "Composer's work" , "Work (of a composer?)" , "Composition - magnum __" , "Work or composition, usually musical" , "A Latin work" .)