Agree to start with friend (5)
I believe the answer is:
tally
'agree' is the definition.
(I know that agree can be written as tally)
'to start with friend' is the wordplay.
'start with' suggests taking the first letters.
'friend' becomes 'ally' (an ally is a friend or supporter).
The first letter of 'to' is 't'.
't'+'ally'='TALLY'
(Other definitions for tally that I've seen before include "Agree - count" , "Total score" , "Mark" , "Record; current score" , "highly?" .)