Terribly neat feature of Sicily (4)
I believe the answer is:
etna
'feature of sicily' is the definition.
(I've seen this before)
'terribly neat' is the wordplay.
'terribly' is an anagram indicator.
'neat' anagrammed gives 'ETNA'.
(Other definitions for etna that I've seen before include "Italian smoker" , "Constant smoker" , "It could erupt" , "Sicily's 15" , "Threatening mountain" .)