Past exposed by cool denial (5)
I believe the answer is:
olden
'past' is the definition.
('olden' can be similar in meaning to 'past')
'exposed by cool denial' is the wordplay.
'exposed by' says the answer is hidden in the clue (I've seen this in other clues).
'OLDEN' is hidden in the letters of 'cool denial'.
(Other definitions for olden that I've seen before include "Bygone" , "times gone by" , "as in time gone by" , "Led on (anag)" , "Out of date" .)