Hammer small volcanic rock (5)
I believe the answer is:
stuff
'hammer small volcanic rock' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe there's a link between them I don't understand?
'small volcanic rock' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'volcanic rock' becomes 'tuff' (I've seen this in another clue).
's'+'tuff'='STUFF'
Can you help me to learn more?
(Other definitions for stuff that I've seen before include "set with unspecified ingredients" , "12 [GLUT]" , "Material - things" , "Material - gorge" , "Substance; material" .)