Cold thinner is sharper (7)
I believe the answer is:
cleaner
'sharper' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'cold thinner' is the wordplay.
'cold' becomes 'C' (eg on taps).
'thinner' becomes 'leaner' ('lean' is associated in meaning with 'thin').
'c'+'leaner'='CLEANER'
'is' is the link.
Can you help me to learn more?
(Other definitions for cleaner that I've seen before include "Charlady" , "Eg, launderer, duster" , "One who dusts or vacuums" , "Char; detergent" , "More sanitary" .)