Crossword Genius

Marry sweetheart behind flat (5)

Ross

I believe the answer is:

unite

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in the Sydney Morning Herald.
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