Second musical yarn (6)
I believe the answer is:
mohair
'yarn' is the definition.
(I've seen this before)
'second musical' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'musical' becomes 'hair' (I've seen this before).
'mo'+'hair'='MOHAIR'
(Other definitions for mohair that I've seen before include "Fabric from coating of angora goat" , "Fine silky coat of angora goat" , "Fabric made from the coat of an Angora goat" , "Yarn provided by goats" , "Fabric made from Angora goat wool" .)