Turn very morose in the end after sweetheart returns (7)
I believe the answer is:
revolve
'turn' is the definition.
(revolving is a kind of turning)
'very morose in the end after sweetheart returns' is the wordplay.
'very' becomes 'v' (abbreviation).
'in the end' suggests the final letters.
'after' means one lot of letters go next to another.
'sweetheart' becomes 'lover' (sweetheart can mean a lover).
'returns' says the letters should be written backwards.
The last letter of 'morose' is 'e'.
'lover' backwards is 'revol'.
'v'+'e'='ve'
've' after 'revol' is 'REVOLVE'.
(Other definitions for revolve that I've seen before include "Turn around" , "Turn on an axis" , "Pirouette" , "Get round" , "Go round" .)