In mature age reveller is keen (5)
I believe the answer is:
eager
'is keen' is the definition.
(I've seen this before)
'in mature age reveller' is the wordplay.
'in' says the answer is hidden in the clue.
'EAGER' can be found hidden inside 'mature age reveller'.
(Other definitions for eager that I've seen before include "Keenly interested" , "Impatient" , "Swiss mountain" , "Zealous" , "Ardent" .)