Mostly painless transitions for King Charles? (7)
I believe the answer is:
spaniel
'king charles?' is the definition.
(King Charles spaniel is a breed of spaniel)
'mostly painless transitions' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'transitions' indicates an anagram.
'painless' with its last letter removed is 'painles'.
'painles' with letters rearranged gives 'SPANIEL'.
'for' acts as a link.
(Other definitions for spaniel that I've seen before include "Dog with long silky coat" , "See 13" , "Type of dog" , "Dog - leaps in (anag)" , "Gundog; obsequious type" .)