Bit of a gash causing a mask to be worn (3)
I believe the answer is:
gas
'be worn' is the definition.
I can't tell whether this definition defines the answer.
'bit of a gash causing a mask' is the wordplay.
'bit of' indicates taking the first letters.
'causing a mask' becomes 'as' (I can't explain this - if you can you should give a lot more credence to this answer).
The initial letter of 'gash' is 'g'.
'g'+'as'='GAS'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for gas that I've seen before include "It's not liquid or solid, like American petrol" , "Gossip" , "idle chit-chat" , "Wind" , "Eg, hydrogen" .)