A burning issue (4)
'issue?' is the definition.
The answer and definition can be both related to change as well as being verbs in their base form.
Maybe they are linked in a way I don't understand?
'a burning' is the wordplay.
'burning' becomes 'sh' (I can't explain this - if you can you should give a lot more credence to this answer).
'a'+'sh'='ASH'
Can you help me to learn more?
(Other definitions for ash that I've seen before include "Runic letter; tree" , "Burnt material; tree" , "What's often left after something 1 Ac" , "Powdery residue from fire or cigarette" , "Common tree" .)
But I've got no explanation for this one! Could you help me understand?
(Other definitions for lava that I've seen before include "Discharge from volcano" , "Flowing volcanic matter" , "Emanating magma" , "Volcanic flowing substance" , "Stuff heatedly hurled out" .)