To agree without reservation (7)
I believe the answer is:
totally
'without reservation' is the definition.
(I've seen this before)
'to agree' is the wordplay.
'agree' becomes 'tally' (synonyms).
'to'+'tally'='TOTALLY'
(Other definitions for totally that I've seen before include "Wholly" , "Allowing no exception" , "to the nth degree" , "Utterly" , "Quite" .)