Secret society joined by a new islander (6)
I believe the answer is:
tongan
'islander' is the definition.
(someone from Tonga)
'secret society joined by a new' is the wordplay.
'secret society' becomes 'tong' (term for a Chinese secret organisation).
'joined by' says to put letters next to each other.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'tong'+'a'+'n'='TONGAN'
(Other definitions for tongan that I've seen before include "One from the Friendly Islands, as was" , "Language" , "Friendly Islander" .)