Marry sweetheart behind flat (5)
I believe the answer is:
unite
'marry' is the definition.
(I've seen this before)
'sweetheart behind flat' is the wordplay.
I cannot quite understand how this works, but
'sweetheart' could be 'e' (the heart/centre of 'sweet') and 'e' is found in the answer.
The remaining letters 'unit' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for unite that I've seen before include "Make one" , "Pull together" , "old coin" , "Join together, merge" , "Come, or join together" .)