Versus a profit, loss finally beginning to tell (7)
I believe the answer is:
against
'versus' is the definition.
(thesaurus)
'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'.
'a'+'gain'+'s'+'t'='AGAINST'
(Other definitions for against that I've seen before include "In disagreement with" , "Anti, touching" , "Con" , "In conflict with" , "Not supporting" .)