Spotted one half-mask (6)
I believe the answer is:
domino
'mask' is the definition.
(domino is a kind of mask)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for domino that I've seen before include "Loose cloak with mask for upper face" , "Black and white game tile" , "Playing piece with spots" , "garment" , "Hooded cloak and eye-mask in masquerade" .)