Heard chap get letters (4)
I believe the answer is:
'letters' is the definition.
(thesaurus)
'heard chap' is the wordplay.
'heard' shows a homophone (sound like).
'chap' becomes 'male' (I've seen this in other clues).
'male' sounds like 'MAIL'.
'get' acts as a link.
(Other definitions for mail that I've seen before include "Form of armour; post" , "Post in America" , "Armour; posted items" , "Paper" , "Dispatch - armour" .)