Turn very regretful at heart after sweetheart's upset (7)
I believe the answer is:
revolve
'turn' is the definition.
(I've seen this before)
'very regretful at heart after sweetheart's upset' is the wordplay.
'very' becomes 'v' (abbreviation).
'at heart' means to look at the middle letters.
'after' is a charade indicator (letters next to each other).
'sweetheart' becomes 'lover' (I've seen this before**).
'upset' is a reversal indicator.
The central letter of 'regretful' is 'e'.
'lover' backwards is 'revol'.
'v'+'e'='ve'
've' put after 'revol' is 'REVOLVE'.
(Other definitions for revolve that I've seen before include "Pirouette" , "Turn round" , "Go round" , "Turn on an axis" , "Get round" .)