One always fuming Brit looms nastily (9)
I believe the answer is:
stromboli
'one always fuming' is the definition.
(Stromboli is an active volcano)
'brit looms nastily' is the wordplay.
'nastily' is an anagram indicator.
'brit'+'looms'='britlooms'
'britlooms' with letters rearranged gives 'STROMBOLI'.
(Other definitions for stromboli that I've seen before include "Active volcano off SW Italy" , "Isle" , "Italian volcano" , "1950 film" , "Volcanic island off Sicily" .)