Resist plan that's relative (6)
I believe the answer is:
sister
'that's relative' is the definition.
(sister is a kind of relative)
'resist plan' is the wordplay.
'plan' indicates anagramming the letters (plan can mean organise).
'resist' with letters rearranged gives 'SISTER'.
(Other definitions for sister that I've seen before include "Female relation" , "Nurse, nun or relation" , "Nurse, nun and sibling" , "Close female relative" , "Resist about the sibling" .)