Brief affair involving Peg recalled (8)
I believe the answer is:
fleeting
'brief' is the definition.
(I've seen this before)
'affair involving peg recalled' is the wordplay.
'affair' becomes 'fling' (a fling is a romantic affair).
'involving' means one lot of letters goes inside another.
'peg' becomes 'tee' (tee is a kind of peg).
'recalled' shows that the letters should be reversed in order (to recall can mean to bring something back).
'tee' back-to-front is 'eet'.
'fling' placed around 'eet' is 'FLEETING'.
(Other definitions for fleeting that I've seen before include "Transient" , "Passing quickly, transitory" , "Ephemeral" , "fast" , "Evanescent" .)