An increase not for the first time (5)
I believe the answer is:
again
'not for the first time' is the definition.
(I've seen this in another clue)
'an increase' is the wordplay.
'an' becomes 'a'.
'increase' becomes 'gain' (gaining is a kind of increasing).
'a'+'gain'='AGAIN'
(Other definitions for again that I've seen before include "Ocne more" , "Anew" , "Furthermore" , "second term" , "return" .)