Cool morning, rejected air conditioning for fan (6)
I believe the answer is:
maniac
'fan' is the definition.
(I know that maniac is a type of fan)
'cool morning rejected air conditioning' is the wordplay.
'cool' becomes 'in' ('in' can mean fashionable or cool).
'morning' becomes 'am' (as in in:00 AM).
'rejected' says the letters should be written backwards.
'air conditioning' becomes 'AC'.
'in'+'am'='inam'
'inam' in reverse letter order is 'mani'.
'mani'+'ac'='MANIAC'
'for' is the link.
(Other definitions for maniac that I've seen before include "Wild, perhaps violent, type" , "Psycho" , "Lunatic or person behaving wildly" , "Insane person" , "Lunatic, madman" .)