Old cave unearthed by instant replay (5)
I believe the answer is:
antre
'old cave' is the definition.
(archaic term for a cave)
'unearthed by instant replay' is the wordplay.
'unearthed by' indicates the answer is hidden within the clue (the answer is hidden or buried within the clue).
'ANTRE' is hidden within 'instant replay'.
(Other definitions for antre that I've seen before include "Formerly, a cavern" , "Cave used by poets" .)