Fish circling cross current's beginning to do well (5)
I believe the answer is:
Here is my best explanation:
'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'.
'eel' enclosing 'xc' is 'EXCEL'.
(Other definitions for excel that I've seen before include "Be the best", "cap", "Do brilliantly", "Outshine", "Perform extremely well", "Do outstandingly well", "Outrival", "Predominate in", "Stand out" and "Be exceptionally good (at a subject)".)