Defenceless? Not 'urt (7)
I believe the answer is:
unarmed
'defenceless?' is the definition.
(not carrying a weapon)
'not 'urt' is the wordplay.
'not' becomes 'un' (prefix meaning 'not').
''urt' becomes ''armed' ('harmed' - I've seen this in another clue - said with a cockney accent, dropping the 'H', like 'hurt' was).
'un'+'armed'='UNARMED'
(Other definitions for unarmed that I've seen before include "Lacking weapons" , "with no warlike threat?" , "Maunder (anag.)" , "Defenceless" , "Not equipped with or carrying weapons" .)