Defeat: head hidden in photo finish? (4)
I believe the answer is:
loss
'defeat' is the definition.
(I've seen this before)
'head hidden in photo finish?' is the wordplay.
'head hidden in' means to remove the first letter.
'photo finish?' becomes 'gloss' (I've seen this in another clue).
'gloss' with its initial letter removed is 'LOSS'.
(Other definitions for loss that I've seen before include "(Sporting) defeat" , "Forgetting" , "Cost" , "Red figures on a balance sheet or ledger" , "(Sports) defeat" .)