Agrees to head military friends (7)
I believe the answer is:
tallies
'agrees' is the definition.
('tally' can be a synonym of 'agree')
'head military friends' is the wordplay.
'head military' becomes 't' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'friends' becomes 'allies' ('ally' can be a synonym of 'friend').
't'+'allies'='TALLIES'
'to' is the link.
Can you help me to learn more?
(Other definitions for tallies that I've seen before include "Agrees; scores" , "Matches, corresponds" , "Reckons debt or score" , "Counts, of votes say" , "Recordings" .)