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' is the subsidiary indication.
'fish' becomes 'eel' (eel is a kind of fish**).
'circling' means one lot of letters goes inside another.
'cross' becomes 'x'.
'beginning' says to take the initial letters.
The initial letter of 'currents' is 'c'.
'eel' going around 'xc' is 'excel'.
'to' is the link.
(Other definitions for excel that I've seen before include "well above average", "Better", "surpass all others", "Stand out", "Be exceptionally good (at a subject)", "cap", "Be very good at, better than others", "Do brilliantly", "Outdo" and "Be outstanding".)