Groom-to-be (6)
I believe the answer is:
fiance
'groom-to-be' is the definition.
(man someone is engaged to)
This is the entire clue.
(Other definitions for fiance that I've seen before include "One's betrothed" , "who may be planning marriage" , "Betrothed man" , "One engaged to marry" , "Prospective groom" .)