Alter into suit (6)
I believe the answer is:
become
I believe this is a double definition.
'alter into' is the first definition.
(thesaurus)
'suit' is the second definition.
('become' can be a synonym of 'suit')
(Other definitions for become that I've seen before include "Grow to be" , "Flatter" , "Develop into something" , "Undergo a change or development" , "Suit; turn into" .)