Flies, to a spider (4)
I believe the answer is:
'flies to a spider' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for prey that I've seen before include "Animal hunted by another for food" , "Quarry, target" , "Hunted creature" , "Animal sought as food" , "Victim, quarry" .)