Naughty sweetheart proposed (4)
I believe the answer is:
bade
'proposed' is the definition.
(I've seen this in another clue)
'naughty sweetheart' is the wordplay.
'naughty' becomes 'bad' (badly behaved).
'sweetheart' becomes 'e' (the middle letter or heart of 'sweet').
'bad'+'e'='BADE'
(Other definitions for bade that I've seen before include "Commanded with bead" , "Ordered" , "Invited" .)