Transient affair involving headless runners? (8)
I believe the answer is:
fleeting
'transient' is the definition.
(I've seen this before)
'affair involving headless runners?' is the wordplay.
'affair' becomes 'fling' (a fling is a romantic affair).
'involving' is an insertion indicator.
'headless' means to remove the first letter.
'runners?' becomes 'feet' (I've seen this in another clue).
'feet' with its initial letter taken off is 'eet'.
'fling' placed around 'eet' is 'FLEETING'.
(Other definitions for fleeting that I've seen before include "Ephemeral" , "Lasting for a brief time" , "Evanescent" , "Passing quickly, transitory" , "Transient" .)