Athenian's dip in defeat, alas, a marathon backfiring (12)
I believe the answer is:
taramasalata
'athenian's dip' is the definition.
'taramasalata' can be an answer for 'dip' (I've seen this before). I also know that 'athenian' relates to this answer.
'in defeat alas a marathon backfiring' is the wordplay.
'in' indicates the answer is hidden within the clue.
'backfiring' is a reversal indicator.
'atalasamarat' is hidden within 'defeat alas a marathon'.
'atalasamarat' in reverse letter order is 'TARAMASALATA'.
(Other definitions for taramasalata that I've seen before include "Pinkish fish roe paste" , "fishy paste" , "spread" , "starter" , "Greek paste made from fish roe" .)