That thing in a mask is an alien (7)
I believe the answer is:
visitor
'an alien' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'that thing in a mask' is the wordplay.
'that thing' becomes 'it'.
'in' means one lot of letters goes inside another.
'a mask' becomes 'visor' (I've seen this before).
'it' inserted inside 'visor' is 'VISITOR'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for visitor that I've seen before include "Company" , "Guest - tourist" , "who might stay?" , "Seasonal migratory bird" , "Person turning up" .)