Powder stock regularly contains aluminium (4)
I believe the answer is:
talc
'powder' is the definition.
(talcing is a kind of powdering)
'stock regularly contains aluminium' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'contains' indicates putting letters inside.
'aluminium' becomes 'al' (Al is the chemical symbol for aluminium).
The alternate letters of 'stock' are 'tc'.
'tc' enclosing 'al' is 'TALC'.
(Other definitions for talc that I've seen before include "Drier" , "It is used to absorb moisture" , "Magnesium silicate powder" , "Powder used as toiletry" , "Toilet powder - mica" .)