Take in or let out (5)
I believe the answer is:
alter
'take in or let out' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for alter that I've seen before include "Bout of extravagant spending" , "Vary" , "Make changes to" , "and 10 Across: Second self" , "convert" .)