Some champers is (tentatively) endless (10)

I believe the answer is:

persistent

'endless' is the definition.
(I've seen this before)

'some champers is tentatively' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'PERSISTENT' can be found hidden inside 'champers is tentatively'.

(Other definitions for persistent that I've seen before include "Dogged" , "Continual" , "fixed" , "Endless" , "Resolute - interminable" .)

I've seen this clue in The Everyman puzzle in The Observer.

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