A spirit boxing ace shows once more (5)
I believe the answer is:
again
'once more' is the definition.
('again' can be a synonym of 'once more')
'a spirit boxing ace' is the wordplay.
'spirit' becomes 'gin' (I've seen this before).
'boxing' means one lot of letters goes inside another ('box' can mean contain, as in to put in a box).
'ace' becomes 'a'.
'gin' enclosing 'a' is 'gain'.
'a'+'gain'='AGAIN'
'shows' is the link.
(Other definitions for again that I've seen before include "Not for the first time" , "Here we go ___" , "Predictably perhaps" , "One more time, encore" , "Furthermore" .)