Sweetheart rejected? Time for rebellion (6)
I believe the answer is:
revolt
'rebellion' is the definition.
(I know that rebellion can be written as revolt)
'sweetheart rejected? time for' is the wordplay.
'sweetheart' becomes 'lover' (sweetheart can mean a lover).
'rejected?' says the letters should be written in reverse.
'time for' becomes 't' (I can't explain this - if you can you should give a lot more credence to this answer).
'lover' back-to-front is 'revol'.
'revol'+'t'='REVOLT'
Can you help me to learn more?
(Other definitions for revolt that I've seen before include "'Uprising, rebellion (6)'" , "Upset" , "Rebel or disgust" , "Rebel, rise up against" , "Organised opposition to authority" .)