Conceals many questions (5)
I believe the answer is:
'conceals' is the definition.
(I've seen this in another clue)
'many questions' is the wordplay.
'many' becomes 'm' (Roman numeral for a thousand).
'questions' becomes 'asks' (as in asking a question).
(Other definitions for masks that I've seen before include "Coverings to conceal the face" , "Hides, disguises" .)