Musical's song describing custom (8)
I believe the answer is:
carousel
'musical's' is the definition.
(I've seen this before)
'song describing custom' is the wordplay.
'song' becomes 'carol' (carol is a kind of song).
'describing' is an insertion indicator (describe can mean to form the outline of).
'custom' becomes 'use' (use is a kind of custom).
'carol' placed around 'use' is 'CAROUSEL'.
(Other definitions for carousel that I've seen before include "Revolving belt for luggage at airport" , "Musical, roundabout" , "baggage carrier" , "Rodgers/Hammerstein hit" , "fair entertainment?" .)