Uncharacteristically fertile places (5)

I believe the answer is:

oases

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'fertile places' is the definition.
(I've seen this in another clue)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for oases that I've seen before include "These sanctuaries" , "Fertile desert spots" , "Fertile desert places" , "Fertile areas in deserts" , "Fertile tracts in a desert" .)

I've seen this clue in the Evening Standard.

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