Relative rung by nurse (10)
I believe the answer is:
stepsister
'relative' is the definition.
(stepsister is a kind of relative)
'rung by nurse' is the wordplay.
'rung' becomes 'step' (I've seen this in another clue).
'by' is a charade indicator (letters next to each other).
'nurse' becomes 'sister' (I've seen this before).
'step'+'sister'='STEPSISTER'
(Other definitions for stepsister that I've seen before include "member of the family" , "one who hated Cinderella" , "Relation gained through a second marriage" , "relative in law" .)