Looking promising rather than thorny (4)

I believe the answer is:

rosy

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'thorny' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for rosy that I've seen before include "lady" , "Blooming - hopeful" , "Blushing - optimistic" , "Pinkish; healthy" , "Healthy-looking" .)

I've seen this clue in the Evening Standard.

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