Seem not unsettled by last trace of whopping shiner? (8)
I believe the answer is:
gemstone
'shiner?' is the definition.
(something which might shine)
'seem not unsettled by last trace of whopping' is the wordplay.
'unsettled' indicates anagramming the letters.
'by' means one lot of letters go next to another.
'last trace of' indicates one should take the final letters.
The last letter of 'whopping' is 'g'.
'seem'+'not'='seemnot'
'seemnot' with letters rearranged gives 'emstone'.
'emstone' after 'g' is 'GEMSTONE'.
(Other definitions for gemstone that I've seen before include "Emerald or ruby" , "Eg, sapphire" , "Eg, diamond or ruby" , "Something precious" , "Physical power" .)