Do well getting 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' means one lot of letters goes inside another.
'ninety' becomes 'XC' (Roman numerals).
'fish' becomes 'eel' (eel is a type of fish).
'xc' put inside 'eel' is 'EXCEL'.
'getting' is the link.
(Other definitions for excel that I've seen before include "Top" , "Be superior to" , "Eclipse" , "Be exceptionally good (at a subject)" , "Be outstanding" .)