He could be both best man and groom (6)

I believe the answer is:

ostler

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'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" .)

I've seen this clue in The Globe and Mail.

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