Shiner's mild swelling (5,4)
I believe the answer is:
light bulb
'shiner's' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'mild swelling' is the wordplay.
'mild' becomes 'light' (I've seen this before).
'swelling' becomes 'bulb' (I've seen this before).
'light'+'bulb'='LIGHT-BULB'
Can you help me to learn more?
(Other definitions for light bulb that I've seen before include "One should make things clear" , "glower" , "(Electric) lamp" , "Source of illumination" , "that'll brighten your surroundings" .)