Suit to change into (6)
I believe the answer is:
become
This clue is a double definition.
'suit' is the first definition.
(becoming can mean fitting or suitable)
'change into' is the second definition.
(I've seen this before)
'to' acts as a link.
(Other definitions for become that I've seen before include "Undergo a change or development" , "Suit, befit" , "Develop into something" , "Suit; turn into" , "Metamorphose into it" .)