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Artfully do a girl a turn in a three-sided relationship (10)

Ross

I believe the answer is:

triangular

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

'in a three-sided relationship' is the definition.
I can't tell whether this defines the answer.

'artfully do a girl a turn' is the wordplay.
'a girl a turn' anagrams to 'TRIANGULAR'.
Though, I'm not sure how the anagram is indicated.

Can you help me to learn more?

(Other definitions for triangular that I've seen before include "Three-sided" , "involving three people?" .)

I've seen this clue in The Times.
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