Application in a letter to us (3)
I believe the answer is:
use
'application' is the definition.
(I've seen this before)
'a letter to us' is the wordplay.
'a letter' becomes 'e' ().
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'e' put after 'us' is 'USE'.
'in' acts as a link.
(Other definitions for use that I've seen before include "Employ (something)" , "consume; exploit" , "Exploit unfairly" , "Consume; deploy" , "Profit" .)