Five old boats around lake - they produce hot stuff (9)
I believe the answer is:
volcanoes
'they produce hot stuff' is the definition.
The definition suggests an adverb but the answer is not.
'five old boats around lake' is the wordplay.
'five' becomes 'V' (Roman numeral).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'boats' becomes 'canoes' (canoeing is a kind of boating).
'around' means one lot of letters goes inside another.
'lake' becomes 'L' (geographical abbreviation).
'o'+'canoes'='ocanoes'
'ocanoes' going around 'l' is 'olcanoes'.
'v'+'olcanoes'='VOLCANOES'
Can you help me to learn more?
(Other definitions for volcanoes that I've seen before include "Erupting fissures in the earth's crust" , "Lava spewers" , "Fissures in the earth's surface that emit lava" , "sources of warm flows" .)