Post for a chap under discussion (4)
I believe the answer is:
'post' is the definition.
(synonyms)
'a chap under discussion' is the wordplay.
'a chap' becomes 'male' (I've seen this in other clues).
'under discussion' indicates a 'sounds like' (homophone) clue.
'male' sounds like 'MAIL'.
'for' is the link.
(Other definitions for mail that I've seen before include "Chain armour" , "letter?" , "Armour of interlinked metal rings" , "Eg, letters" , "Letters and parcels" .)