A fiery mount (4)
I believe the answer is:
etna
'a fiery mount' is the definition.
'Etna' can be an answer for 'mount' (Etna is an example). I'm not certain of the 'a fiery' bit.
'a fiery mount' is the wordplay.
'a' becomes 'an'.
'fiery' becomes 'te' (I can't explain this - if you can you should believe this answer much more).
'mount' says the letters should be written backwards (I've seen 'mounting' mean this (going up in a down clue)).
'an'+'te'='ante'
'ante' reversed gives 'ETNA'.
Can you help me to learn more?
(Other definitions for etna that I've seen before include "Heating vessel" , "Sicilian hothead?" , "Active European volcano" , "Highest active volcano in Europe" , "Neat volcano overturned" .)