About that mask - it's frightening (6)
I believe the answer is:
threat
'frightening' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'about that mask it's' is the wordplay.
'about' indicates putting letters inside.
'mask' becomes 'r' (I can't justify this - if you can you should give a lot more credence to this answer).
'it' becomes 'e' ('e' can mean 'electronic' which is similar to 'IT').
'r'+'e'='re'
'that' enclosing 're' is 'THREAT'.
Can you help me to learn more?
(Other definitions for threat that I've seen before include "An intimidatory warning" , "Menace of impending evil" , "Possibility of danger" , "Statement of intent to do harm" , "Potential source of danger" .)