Settler, one wearing strange garment (8)
I believe the answer is:
emigrant
'settler' is the definition.
(thesaurus)
'one wearing strange garment' is the wordplay.
'one' becomes 'i' (Roman numeral).
'wearing' indicates putting letters inside.
'strange' indicates an anagram.
'garment' with letters rearranged gives 'emgrant'.
'i' put within 'emgrant' is 'EMIGRANT'.
(Other definitions for emigrant that I've seen before include "I've left home for good" , "One leaving for another country" , "Write carelessly" , "one on the way out?" , "Time Gran went abroad to settle" .)