It makes a brief appearance when one snaps (8)
I believe the answer is:
aperture
'it makes a brief appearance when one snaps' is the definition.
The definition suggests a singular noun which matches the answer.
'it makes a brief appearance when one' is the wordplay.
I cannot really understand how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
'a' could be 'per' and 'per' is found within the answer.
'when' could be 'e' (I've seen this in other clues) and 'e' is located in the answer.
'one' could be 'a' and 'a' is found within the answer.
The remaining letters 'ur' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for aperture that I've seen before include "Crack" , "Device controlling amount of light admitted" , "Opening (for a photographer?)" , "Small hole or opening" , "Opening, mostly small" .)