Corresponds, as with a letter to friends (7)
I believe the answer is:
tallies
'corresponds' is the definition.
('tally' can be a synonym of 'correspond')
'with a letter to friends' is the wordplay.
'with' means one lot of letters go next to another.
'a letter' becomes 't' ().
'to friends' becomes 'allies' ('ally' can be a synonym of 'friend').
't' put next to 'allies' is 'TALLIES'.
'as' is the link.
(Other definitions for tallies that I've seen before include "Agrees; scores" , "Recordings" , "Matches, corresponds" , "Sums up" , "Reckons debt or score" .)