The old lady one's hurt (4)
I believe the answer is:
maim
'hurt' is the definition.
(I know that maim is a type of injure)
'the old lady one's' is the wordplay.
'the old lady' becomes 'ma' (both can mean mother).
'one's' becomes 'I'm' ('one is' is a formal way of saying 'I am').
'ma'+'im'='MAIM'
(Other definitions for maim that I've seen before include "Wound so that part of body is permanently damaged" , "Wound and cripple" , "'Disable, mutilate (4)'" , "Injure or harm severely" , "Injure seriously, leaving permanent damage" .)