Mass demand for protective covering (4)
I believe the answer is:
mask
'protective covering' is the definition.
(mask is a kind of protective covering)
'mass demand' is the wordplay.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'demand' becomes 'ask' (synonyms).
'm'+'ask'='MASK'
'for' acts as a link.
(Other definitions for mask that I've seen before include "Cover for the face" , "Domino" , "cloak" , "Conceal" , "Face covering; disguise" .)