Tricky; finished struggling (8)

I believe the answer is:

fiendish

I'm a little stuck... Click here to teach me more about this clue!

'struggling' is the definition.
Both the answer and definition are adjectives. Maybe there's a link between them I don't understand?

'tricky finished' is the wordplay.
'tricky' is an anagram indicator.
'finished' with letters rearranged gives 'FIENDISH'.

Can you help me to learn more?

(Other definitions for fiendish that I've seen before include "Diabolic" , "Infernal" , "Diabolical" , "like some Sudokus" , "Terrible" .)

I've seen this clue in The Everyman puzzle in The Observer.

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