Happens not to be the thing tinned? The opposite (4)

I believe the answer is:

isnt

'happens not' is the definition.
(is not)

I cannot understand how the remainder of the clue works.

(Other definitions for isnt that I've seen before include "Ain't" , "Does not happen to be" , "denial?" , "Is not (colloq.)" , "In the present, not to be" .)

I've seen this clue in The Times.

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