Perform with former sweetheart? Sweet! (7)
I believe the answer is:
execute
'perform' is the definition.
(synonyms)
'with former sweetheart? sweet' is the wordplay.
'with' says to put letters next to each other.
'former' becomes 'ex' (prefix meaning former or onetime).
'sweetheart?' becomes 'e' (the heart/centre of 'sweet').
'sweet' becomes 'cute' (I've seen this before).
'ex' put next to 'e' is 'exe'.
'exe'+'cute'='EXECUTE'
(Other definitions for execute that I've seen before include "Put to death - put into effect" , "Put (plan) into effect; kill (a condemned person)" , "Carry into effect" , "Complete - implement" , "ice" .)