To agree completely (7)
I believe the answer is:
totally
'completely' is the definition.
(I know that completely can be written as totally)
'to agree' is the wordplay.
'agree' becomes 'tally' (synonyms).
'to'+'tally'='TOTALLY'
(Other definitions for totally that I've seen before include "Dead" , "without reservation" , "in every respect" , "without reserve" , "Quite" .)