In middle-age really keen (5)
I believe the answer is:
eager
'keen' is the definition.
(I've seen this before)
'in middle-age really' is the wordplay.
'in' indicates the answer is hidden within the clue.
'EAGER' can be found hidden inside 'middle age really'.
(Other definitions for eager that I've seen before include "Enthusiastic, avid" , "Keen - enthusiastic" , "Impatient" , "Keen (as a beaver?)" , "Anxious" .)