Best work is before setter's filled stomach (7)
I believe the answer is:
optimum
'best' is the definition.
(similar in meaning)
'work is before setter's filled stomach' is the wordplay.
I cannot really see how this works, but
'work' could be 'op' (common abbreviation for 'opus') and 'op' is found in the answer.
'setter's' could be 'im' (I've seen this in other clues) and 'im' is present in the answer.
'stomach' could be 'tum' (synonyms) and 'tum' is found within the remaining letters.
No letters remain.
This explanation may well be incorrect...
(Other definitions for optimum that I've seen before include "model" , "Most favourable (conditions)" , "Most appropriate" , "Best, most favourable" , "Best (result)" .)