Artfully do a girl a turn in a three-sided relationship (10)
I believe the answer is:
triangular
'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?" .)