Such letters are presumably in the mail (5)
I believe the answer is:
chain
'mail' is the definition.
(synonyms)
'such letters are presumably in' is the wordplay.
I cannot quite understand how this works, but
'are' could be 'a' (short for 'are', historical unit of measurement) and 'a' is present in the answer.
'in' is present in the answer.
The remaining letters 'ch' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'the' is the link.
Can you help me to learn more?
(Other definitions for chain that I've seen before include "a number of hotels" , "several supermarkets" , "Group of atoms" , "Connected sequence" , "lots of shops?" .)