Many call for cover (4)
I believe the answer is:
mask
'cover' is the definition.
(masking is a kind of covering)
'many call for' is the wordplay.
'many' becomes 'm' (Roman numeral for a thousand).
'call for' becomes 'ask' (synonyms).
'm'+'ask'='MASK'
(Other definitions for mask that I've seen before include "Face-cover" , "Camouflage" , "Screen" , "One to hide face" , "Face-covering" .)