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" .)

I've seen this clue in The Irish Independent, The Times, the Evening Standard and The Globe and Mail.

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