Volunteers reach island, one in the south Pacific (6)
I believe the answer is:
tahiti
'one in the south pacific' is the definition.
(Pacific island)
'volunteers reach island' is the wordplay.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'reach' becomes 'hit' (synonyms).
'island' becomes 'i'.
'ta'+'hit'+'i'='TAHITI'
(Other definitions for tahiti that I've seen before include "Pacific island, Gauguin's home in the 1890s" , "Polynesian island where Gauguin lived" , "I hit at this exotic island" , "Romantic Polynesian island, capital Papeete" , "Polynesian island, chief town Papeete" .)