Confound Number Ten ultimately with ... (7)

I believe the answer is:

nonplus

'confound' is the definition.
(I know that nonplus is a more specific form of the action confound)

'number ten ultimately with' is the wordplay.
'number' becomes 'no' (abbreviation for 'number').
'ultimately' indicates one should take the final letters.
'with' becomes 'plus'.
The last letter of 'ten' is 'n'.
'no'+'n'+'plus'='NONPLUS'

(Other definitions for nonplus that I've seen before include "Disconcert, confound" , "Utterly confuse" , "Confound, bewilder" , "State of confusion" , "Perplex, confound" .)

I've seen this clue in The Telegraph.

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