Agree by letter to a friend (5)
I believe the answer is:
tally
'agree' is the definition.
(I know that agree can be written as tally)
'by letter to a friend' is the wordplay.
'by' means one lot of letters go next to another.
'letter' becomes 't' ().
'to a friend' becomes 'ally' (an ally is a friend or supporter).
't' put next to 'ally' is 'TALLY'.
(Other definitions for tally that I've seen before include "Match - score" , "Agree - count" , "Reckoning of a debt or of votes" , "Count up; match" , "highly?" .)