This volcano puts out when hot (3)
I believe the answer is:
ash
'this volcano puts out' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each 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'
Can you help me to learn more?
(Other definitions for ash that I've seen before include "Part of forest" , "Oleaceous tree" , "Rowan tree is also called the mountain . . ." , "Silvery-grey" , "Tree with silver-grey bark" .)