To agree entirely (7)
I believe the answer is:
totally
'entirely' is the definition.
('totally' can be a synonym of 'entirely')
'to agree' is the wordplay.
'agree' becomes 'tally' ('tally' can be a synonym of 'agree').
'to'+'tally'='TOTALLY'
(Other definitions for totally that I've seen before include "Completely, entirely" , "Clean" , "to the nth degree" , "To the full extent" , "Fully, completely" .)