Love affair to fill hollow in middle-age (5)
I believe the answer is:
fling
'love affair' is the definition.
(I've seen this in other clues)
'fill hollow in middle-age' is the wordplay.
'hollow' means to remove the middle letters.
'middle' says to take the centre.
'fill' with its centre removed is 'fl'.
The central letter of 'age' is 'g'.
'fl'+'in'+'g'='FLING'
'to' is the link.
(Other definitions for fling that I've seen before include "Brief amour; throw" , "Try" , "Toss" , "extramarital affair" , "Vigorous Scottish dance" .)