Groom is sure to get married at one (9)
I believe the answer is:
stableman
'groom' is the definition.
(synonyms)
'sure to get married at one' is the wordplay.
'sure' becomes 'stable'.
'to get' means one lot of letters go next to another.
'married' becomes 'm'.
'at' means one lot of letters go next to another.
'one' becomes 'an' (I've seen this in other clues).
'stable'+'m'+'an'='STABLEMAN'
'is' is the link.
(Other definitions for stableman that I've seen before include "Maybe horse trainer" , "Ostler, groom etc" .)