When hot it's put out by volcano (3)
I believe the answer is:
ash
'put out by volcano' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'when hot' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'hot' becomes 'h' (abbreviation eg on taps).
'as'+'h'='ASH'
'it's' is the link.
Can you help me to learn more?
(Other definitions for ash that I've seen before include "What's left after burning" , "It's used for flooring" , "Oleaceous tree" , "Dusty residue of a fire" , "Wooden" .)