Busy preparing for matrimony (7)
I believe the answer is:
engaged
'busy' is the definition.
(similar in meaning)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for engaged that I've seen before include "Betrothed - busy" , "prior to one's marriage" , "Bound by promise" , "Given a ring" , "looking forward to the match?" .)