Agree by letter to a friend (5)
I believe the answer is:
Here is my best explanation:
'agree' is the definition.
('tally' can be a synonym of 'agree')
'letter to a friend' is the subsidiary indication.
'letter' becomes 't' (**).
'to' says to put letters next to each other.
'a friend' becomes 'ally' ('ally' can be a synonym of 'friend').
'by' is the link.
(Other definitions for tally that I've seen before include "highly?", "Current score; agree", "Square", "Count, add up", "Match - score", "Total score", "Agree - count", "Running total", "Score; correspond" and "Reckon up".)