Grown apart, as not suitable (5)
I believe the answer is:
wrong
'not suitable' is the definition.
'grown apart' is the wordplay.
'apart' indicates an anagram.
'grown' is an anagram of 'WRONG'.
'as' is the link.
(Other definitions for wrong that I've seen before include "Incorrect, illegal or immoral" , "Treat unjustly" , "bad" , "Erroneous" , "Injury - incorrect" .)