Element of upholstery upset mother of French-polisher? (4,6)
I believe the answer is:
foam rubber
'element of upholstery' is the definition.
(material used in upholstery)
'upset mother of french-polisher?' is the wordplay.
'upset' says the letters should be written in reverse.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'french-polisher?' becomes 'rubber' (someone using French polish).
'ma'+'of'='maof'
'maof' reversed gives 'foam'.
'foam'+'rubber'='FOAM-RUBBER'
(Other definitions for foam rubber that I've seen before include "Filling for cushions etc" , "Common cushion filler" .)