Piggy-bank feature (4)
I believe the answer is:
slot
'piggy-bank feature' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for slot that I've seen before include "Long narrow aperture" , "Opening (for coins)" , "Long narrow opening" , "Narrow opening as in gaming machine" , "Long narrow slit" .)