Doubt the purpose of English Heritage is losing power (11)

I believe the answer is:

reservation

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'doubt' is the definition.
(reservation is a kind of doubt)

I don't understand the remainder of the clue.

(Other definitions for reservation that I've seen before include "misgiving" , "Qualification, proviso" , "Booking - qualification" , "Expression of doubt; table booking" , "Putting aside" .)

I've seen this clue in The Times.

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