Western pianist and chemist's footwear (10,5)
I believe the answer is:
wellington boots
'footwear' is the definition.
(I've seen this before)
'western pianist and chemist's' is the wordplay.
'western' becomes 'w'.
'pianist' becomes 'ellington' (jazz pianist Duke Ellington).
'and' says to put letters next to each other.
'chemist' becomes 'boots' (chain of UK chemists).
'w'+'ellington'+'boots'='WELLINGTON BOOTS'
(Other definitions for wellington boots that I've seen before include "Footwear" , "rubber items? - with 24a" , "calves protected by these?" .)