He could be both best man and groom (6)
I believe the answer is:
'groom' is the definition.
(I know that groom can be written as ostler)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for ostler that I've seen before include "Inn stableman (hist.)" , "Groom at inn" , "ER lost (anag) - former stableman" , "Formerly a stableman" , "Groom, stableman" .)