Post a letter at last (4)
I believe the answer is:
send
'post' is the definition.
(synonyms)
'a letter at last' is the wordplay.
'a letter' becomes 's' ().
'at' says to put letters next to each other.
'last' becomes 'end' (I've seen this in another clue).
's'+'end'='SEND'
(Other definitions for send that I've seen before include "Transport" , "Forward" , "broadcast" , "Mail" , "Sentence to a prison term" .)