Cut into side that's getting bigger (6)
I believe the answer is:
waxing
'cut' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'side that's getting bigger' is the wordplay.
'side' becomes 'wing' (as in left and right wings).
'that's getting' is an insertion indicator.
'bigger' becomes 'ax' (I can't explain this - if you can you should give a lot more credence to this answer).
'wing' placed around 'ax' is 'WAXING'.
'into' acts as a link.
Can you help me to learn more?
(Another definition for waxing that I've seen is " (Of moon) increasing".)