Theatre ultimately invested in opening, not ending (9)

I believe the answer is:

unceasing

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'theatre ultimately invested in opening' is the definition.
I can't tell whether this definition defines the answer.

'not ending' is the wordplay.
'not' becomes 'un' (I've seen this in other clues).
'ending' becomes 'ceasing' ('cease' can be a synonym of 'end').
'un'+'ceasing'='UNCEASING'

Can you help me to learn more?

(Other definitions for unceasing that I've seen before include "Never-ending" , "Endless" , "Continuous" , "Never ending" , "Constant" .)

I've seen this clue in The Telegraph.

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