In same class as relative (6)
I believe the answer is:
sister
'relative' is the definition.
(I know that sister is a type of relative)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for sister that I've seen before include "Female relation" , "habits' - one who has them" , "Senior nurse; relative" , "Tender" , "Resist (anag) - nurse" .)