Union fastening device we had on top (7)
I believe the answer is:
wedlock
'union' is the definition.
(I know that union can be written as wedlock)
'fastening device we had on top' is the wordplay.
'fastening device' becomes 'lock' (I've seen this before).
'we had' becomes 'we'd' (contraction of 'we had').
'on top' means one lot of letters go next to another (in a down clue, some letters go above others).
'lock' put after 'wed' is 'WEDLOCK'.
(Other definitions for wedlock that I've seen before include "Marital state" , "The married state - throw away the key" , "A state of union" , "Matrimony" , "State of marriage" .)