Mount Etna deposits somewhat academic (6)
I believe the answer is:
pedant
'academic' is the definition.
(I've seen this in another clue)
'mount etna deposits somewhat' is the wordplay.
'mount' shows that the letters should be reversed in order (I've seen 'mounting' mean this (going up in a down clue)).
'somewhat' indicates a hidden word.
'tnadep' can be found hidden inside 'etna deposits'.
'tnadep' back-to-front is 'PEDANT'.
(Other definitions for pedant that I've seen before include "Panted (anag.)" , "quibbler" , "Old teacher" , "A sticklet for formal rules and details" , "One concerned with excessive detail" .)