Volunteers reach one Pacific island (6)
I believe the answer is:
tahiti
'pacific island' is the definition.
(I've seen this before)
'volunteers reach one' is the wordplay.
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
'reach' becomes 'hit' ('hit' can be a synonym of 'reach').
'one' becomes 'i' (Roman numeral).
'ta'+'hit'+'i'='TAHITI'
(Other definitions for tahiti that I've seen before include "French Polynesian island" , "South Pacific island where both Robert Louis Stevenson and Paul Gauguin spent time" , "An isle in the South Seas" , "South Pacific island, linked to Paul Gauguin and Robert Louis Stevenson" , "part of Polynesia" .)