Part of Australia unpopular with defender (7)
I believe the answer is:
outback
'part of australia' is the definition.
('outback' is part of 'australia')
'unpopular with defender' is the wordplay.
'unpopular' becomes 'out' ('out' can be similar in meaning to 'unpopular').
'with' is a charade indicator (letters next to each other).
'defender' becomes 'back' (I've seen this in other clues).
'out'+'back'='OUTBACK'
(Other definitions for outback that I've seen before include "Bush country of Australia" , "Australian wilderness" , "Remote land in Australia" , "continental interior?" , "remote region" .)