Island nation taking part in assault on Gallipoli (5)
I believe the answer is:
tonga
'island nation' is the definition.
(country made up of several islands)
'part in assault on gallipoli' is the wordplay.
'part in' indicates a hidden word.
'TONGA' can be found hidden inside 'assault on gallipoli'.
'taking' acts as a link.
(Other definitions for tonga that I've seen before include "Two-wheeled Indian vehicle" , "SW Pacific monarchy" , "Pacific island state" , "South Pacific kingdom, capital Nuku'alofa" , "Pacific Ocean monarchy" .)