Stupid MP in tempestuous isle (6)
I believe the answer is:
simple
'stupid' is the definition.
(thesaurus)
'mp in tempestuous isle' is the wordplay.
'in' means one lot of letters goes inside another.
'tempestuous' indicates an anagram.
'isle' is an anagram of 'sile'.
'mp' going into 'sile' is 'SIMPLE'.
(Other definitions for simple that I've seen before include "The sort of interest" , "Uncomplicated; herb" , "Easily done/understood" , "Impels (anag.)" , "A piece of cake" .)