Mail opening (4)
I believe the answer is:
slot
'mail opening' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for slot that I've seen before include "Lots (anag.)" , "Narrow opening as in gaming machine" , "Small opening, for a coin say" , "This gaming machine operates by insertion of coin" , "Hole for e.g. coin" .)