A trap, covering ace once more (5)
I believe the answer is:
again
'once more' is the definition.
('again' can be a synonym of 'once more')
'a trap covering ace' is the wordplay.
'trap' becomes 'gin' (I've seen this before).
'covering' is an insertion indicator.
'ace' becomes 'a'.
'gin' placed around 'a' is 'gain'.
'a'+'gain'='AGAIN'
(Other definitions for again that I've seen before include "return" , "Predictably perhaps" , "Ocne more" , "second term" , "Anew" .)