On the outside (8)
(Other definitions for exterior that I've seen before include "Foreign" , "Outermost" , "covering" , "Retire ox (anag)" , "Outside" .)
'on the outside' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for external that I've seen before include "Belonging to the outside" , "abroad" , "Outer" , "Lying outside" , "Relating to the outside" .)