Black alloy, cool, turned yellow when exposed (6)
I believe the answer is:
niello
'black alloy' is the definition.
The definition suggests a singular noun which matches the answer.
'cool turned yellow when exposed' is the wordplay.
'cool' becomes 'in' ('in' can mean fashionable or cool).
'turned' says the letters should be written backwards.
'when exposed' says to take the centre (the inside letters become exposed).
The middle of 'yellow' is 'ello'.
'in' reversed gives 'ni'.
'ni'+'ello'='NIELLO'
Can you help me to learn more?
(Other definitions for niello that I've seen before include "Black filling for engravings on metal; lo line (anag.)" , "Style of ornamenting metal, lo line (anag.)" , "Method of engraving" .)