Agree to write (10)
I believe the answer is:
correspond
I believe this is a double definition.
'agree' is the first definition.
(I know that agree is a kind of agree)
'write' is the second definition.
(to correspond is to exchange letters with someone)
'to' acts as a link.
(Other definitions for correspond that I've seen before include "answer" , "email, say?" , "Have similar functions" , "Have a close similarity" , "Write - match" .)