Grow into suit (6)
I believe the answer is:
become
I believe this is a double definition.
'grow into' is the first definition.
(I've seen this in another clue)
'suit' is the second definition.
(becoming can mean fitting or suitable)
(Other definitions for become that I've seen before include "Metamorphose into it" , "Undergo a change or development" , "Grow to be" , "Flatter" , "Suit; turn into" .)