A 'suit' to change into! (6)
I believe the answer is:
become
I believe this is a double definition.
'a suit' is the first definition.
(thesaurus)
'change into' is the second definition.
(thesaurus)
'to' acts as a link.
(Other definitions for become that I've seen before include "Grow to be" , "Flatter" , "Evolve into; suit" , "Metamorphose into it" , "Suit; turn into" .)