Invoice about right for fish? (5)
I believe the answer is:
brill
'for fish?' is the definition.
(brill is a kind of fish)
'invoice about right' is the wordplay.
'invoice' becomes 'bill' (synonyms).
'about' indicates putting letters inside.
'right' becomes 'r' (common abbreviation).
'bill' enclosing 'r' is 'BRILL'.
(Other definitions for brill that I've seen before include "Fab -- fish" , "swimmer" , "Very good - fish" , "Great" , "catch, perhaps" .)