Stuff mum mostly put back (8)
I believe the answer is:
material
'stuff' is the definition.
('material' can be a synonym of 'stuff')
'mum mostly put back' is the wordplay.
'mum' becomes 'mater' (old-fashioned term for mother).
'mostly' means to remove the last letter.
'put' becomes 'laid' (I've seen this before).
'back' is a reversal indicator.
'laid' with its final letter taken off is 'lai'.
'lai' written backwards gives 'ial'.
'mater'+'ial'='MATERIAL'
(Other definitions for material that I've seen before include "grave" , "Important; fabric" , "I am alert to find cloth or information" , "I alert ma (anag.)" , "Directly relevant to a matter, in a law case perhaps" .)