Agree to keep in touch (10)
I believe the answer is:
correspond
This clue is a double definition.
'agree' is the first definition.
(synonyms)
'keep in touch' is the second definition.
(I've seen this before)
'to' is the link.
(Other definitions for correspond that I've seen before include "Exchange letters" , "Have a close similarity" , "answer" , "Have similar functions" , "Swap letters" .)