Mail (4)
'mail' is the definition.
(I know that mail can be written as post)
This is the entire clue.
(Other definitions for post that I've seen before include "employment" , "trading establishment" , "(Holiday) communication" , "Job; upright timber" , "Mail; office" .)
(Other definitions for slot that I've seen before include "Hole for e.g. coin" , "Long narrow aperture" , "Opening for coin or TV programme" , "Long narrow slit" , "Space for coin or programme" .)
'mail' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for letters that I've seen before include "Correspondence" , "...as, for example" , "A, B, C, D etc" , "They may indicate a degree" , "Literary culture" .)
(Other definitions for postal that I've seen before include "Relating to mail delivery" , "Sort of vote" , "To do with snail mail" , "to go so mad in America" , "Relating to mail service" .)
(Other definitions for snail that I've seen before include "go very slowly" , "Slow slithery animal" , "Sluggish person - kind of mail" , "Proverbially slow-moving mollusc" , "Not one of the fleet" .)