Warm up again (6)
I believe the answer is:
reheat
'warm up again' is the definition.
(to heat or warm up again)
This is all the clue.
(Other definitions for reheat that I've seen before include "Cook again" , "Of cooked food, give another go in the microwave" , "Jet engine afterburner" , "Process increasing jet engine thrust" , "Method of getting extra power from jet engine" .)