Once more putting ace over net (5)
I believe the answer is:
again
'once more' is the definition.
(synonyms)
'ace over net' is the wordplay.
'ace' becomes 'a' (abbreviation).
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'net' becomes 'gain' (I can't explain this - if you can you should give a lot more credence to this answer).
'a'+'gain'='AGAIN'
'putting' is the link.
Can you help me to learn more?
(Other definitions for again that I've seen before include "On the other hand" , "Predictably perhaps" , "Repeatedly" , "'Another time, once more (5)'" , "Not for the first time" .)