Old flame last to leave with nicely sexual effect (7)
I believe the answer is:
execute
'effect' is the definition.
(executing is a kind of effecting)
'old flame last to leave with nicely sexual' is the wordplay.
'old flame' becomes 'ex' (a former partner).
'last to' says to take the final letters.
'with' means one lot of letters go next to another.
'nicely sexual' becomes 'cute' (I am not sure about this - if you are sure you should believe this answer much more).
The last letter of 'leave' is 'e'.
'ex'+'e'+'cute'='EXECUTE'
Can you help me to learn more?
(Other definitions for execute that I've seen before include "Off with his head" , "Realise" , "Carry output to death" , "Discharge" , "shoot" .)