A profit once more (5)
I believe the answer is:
again
'once more' is the definition.
(synonyms)
'a profit' is the wordplay.
'profit' becomes 'gain' (synonyms).
'a'+'gain'='AGAIN'
(Other definitions for again that I've seen before include "return" , "Next time" , "'Another time, once more (5)'" , "Anew" , "On the other hand" .)