Mum retails recycled stuff builder needs? (9)
I believe the answer is:
materials
'stuff builder needs?' is the definition.
'mum retails recycled' is the wordplay.
'mum' becomes 'ma' ('ma' can be a synonym of 'mum').
'recycled' indicates anagramming the letters.
'retails' with letters rearranged gives 'terials'.
'ma'+'terials'='MATERIALS'
(Other definitions for materials that I've seen before include "See 10" , "Substances that go into the making something" , "textiles" , "Items needed to tackle a job" .)