Mail (4)
'mail' is the definition.
(synonyms)
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 before)
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" .)