Do well, taking about ninety fish (5)
I believe the answer is:
excel
'do well' is the definition.
(I've seen this before)
'about ninety fish' is the wordplay.
'about' is an insertion indicator.
'ninety' becomes 'xc' (roman numeral).
'fish' becomes 'eel' (eel is a kind of fish**).
'xc' inserted within 'eel' is 'EXCEL'.
'taking' acts as a link.
(Other definitions for excel that I've seen before include "cap" , "Outdo" , "Eclipse" , "Be pre-eminent" , "Do really well, outshine others" .)