Fish circling cross current's beginning to do well (5)
I believe the answer is:
excel
'do well' is the definition.
(I've seen this before)
'fish circling cross current's beginning to' is the wordplay.
'fish' becomes 'eel' (eel is a kind of fish**).
'circling' is an insertion indicator (circling can mean going around or containing).
'cross' becomes 'x'.
'beginning to' indicates taking the first letters.
The initial letter of 'currents' is 'c'.
'x'+'c'='xc'
'eel' enclosing 'xc' is 'EXCEL'.
(Other definitions for excel that I've seen before include "Be outstandingly proficient" , "Be pre-eminent" , "surpass all others" , "Do brilliantly" , "Be exceptionally good (at a subject)" .)