Warning notice: face cover finally mandatory (8)
I believe the answer is:
advisory
'warning' is the definition.
(I've seen this in another clue)
'notice face cover finally mandatory' is the wordplay.
'notice' becomes 'ad' (short for advert).
'face cover' becomes 'visor' (visor can mean a mask).
'finally' says to take the final letters.
The last letter of 'mandatory' is 'y'.
'ad'+'visor'+'y'='ADVISORY'
(Other definitions for advisory that I've seen before include "Recommended but not compulsory" , "Making recommendations" , "Recommending" , "helping" .)