A musical affair? (8)
I believe the answer is:
triangle
This might be a double definition.
'a musical' is the first definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'affair?' is the second definition.
(thesaurus)
Can you help me to learn more?
(Other definitions for triangle that I've seen before include "Gable shape -- 1 across (anag)" , "Large tin (anag.)" , "It has three sides" , "Three-sided figure (maths)" , "Integral part of common geometric figure" .)