An improvement once more... (5)
I believe the answer is:
again
'once more' is the definition.
(synonyms)
'an improvement' is the wordplay.
'an' becomes 'a'.
'improvement' becomes 'gain' (I've seen this in another clue).
'a'+'gain'='AGAIN'
(Other definitions for again that I've seen before include "return" , "Moreover - a second time" , "Ocne more" , "One more time, encore" , "Furthermore" .)