Some big reward got bigger (4)
I believe the answer is:
grew
'got bigger' is the definition.
(I've seen this before)
'some big reward' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'GREW' is hidden in the letters of 'big reward'.
(Other definitions for grew that I've seen before include "Got larger" , "Sprouted" , "Cultivated'" , "Multiplied" , "Got bigger" .)