Not for the first time, an increase (5)
I believe the answer is:
again
'not for the first time' is the definition.
(another time)
'an increase' is the wordplay.
'an' becomes 'a'.
'increase' becomes 'gain' (gain is a kind of increase).
'a'+'gain'='AGAIN'
(Other definitions for again that I've seen before include "On the other hand" , "Here we go ___" , "'Another time, once more (5)'" , "repeat order" , "return" .)