Agreed before leaving, then fell asleep (6,3)
I believe the answer is:
nodded off
'fell asleep' is the definition.
('nod off' can be a synonym of 'fall asleep')
'agreed before leaving' is the wordplay.
'agreed' becomes 'nodded' (I've seen this before).
'before' means one lot of letters go next to another.
'leaving' becomes 'off' (as in 'I'm off').
'nodded'+'off'='NODDED OFF'
'then' acts as a link.
(Other definitions for nodded off that I've seen before include "Fell asleep" , "dozed" .)