Opens up and one gets in with difficulty (7)

I believe the answer is:

straits

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'difficulty' is the definition.
(I know that straits is a type of situation)

I can't explain the rest of the clue.

(Other definitions for straits that I've seen before include "Difficulties" , "Narrow channels of the sea joining two larger bodies of water" , "Sea passages" , "... of Magellan, Hormuz" , "Dire --, rock band" .)

I've seen this clue in the Evening Standard.

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