Versus a profit, loss finally beginning to tell (7)
I believe the answer is:
'versus' is the definition.
'a profit loss finally beginning to tell' is the wordplay.
'profit' becomes 'gain' ('gain' can be a synonym of 'profit'**).
'finally' says to take the final letters.
'beginning to' says to take the initial letters.
The last letter of 'loss' is 's'.
The initial letter of 'tell' is 't'.
(Other definitions for against that I've seen before include "In disagreement with" , "Anti, touching" , "Con" , "In conflict with" , "Not supporting" .)