Good man to fill difficult role as groom (6)
I believe the answer is:
ostler
'groom' is the definition.
(I know that groom can be written as ostler)
'good man to fill difficult role' is the wordplay.
'good man' becomes 'st' (abbreviation for saint).
'to fill' indicates putting letters inside.
'difficult' indicates an anagram.
'role' with letters rearranged gives 'oler'.
'st' put within 'oler' is 'OSTLER'.
'as' is the link.
(Other definitions for ostler that I've seen before include "Attendant for horses in olden times" , "He held the horse at the old inn" , "Horseman at the inn of old" , "Inn's stableman, of old" , "ER lost (anag) - former stableman" .)