Former sweetheart enters punctually (3-4)
I believe the answer is:
one-time
'former' is the definition.
(I know that former can be written as one-time)
'sweetheart enters punctually' is the wordplay.
'sweetheart' becomes 'e' (the middle letter or heart of 'sweet').
'enters' means one lot of letters goes inside another.
'punctually' becomes 'on time' (I've seen this in another clue).
'e' put inside 'ontime' is 'ONE-TIME'.
(Other definitions for one-time that I've seen before include "Relating to a single occasion" , "Former" .)