Warmest season (6)
I believe the answer is:
summer
'warmest season' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for summer that I've seen before include "Would one count" , "'Shall I compare thee to a . . . . . .'s day?' (Shakespeare)" , "Hot season" , "'Now is the winter of our discontent / Made glorious ... by this sun of York' (Richard III)" , "Period of heat" .)