Wearer of shell suit with an extremely unusual design (8)
I believe the answer is:
nautilus
'wearer of shell' is the definition.
(type of mollusc)
'suit with an extremely unusual design' is the wordplay.
'with' means one lot of letters go next to another.
'an extremely' says to hollow out the word (remove centre letters).
'design' is an anagram indicator.
'unusual' with its middle taken out is 'unal'.
'suit' after 'unal' is 'unalsuit'.
'unalsuit' with letters rearranged gives 'NAUTILUS'.
(Other definitions for nautilus that I've seen before include "Cephalopod mollusc" , "Nemo's vessel" , "Usual tin for a small sea creature" , "Sea creature to be found in the usual tin, unusually" , "Sea creature - literary submarine" .)