Resist plan by relation (6)
I believe the answer is:
sister
'relation' is the definition.
(sister is a kind of relation)
'resist plan' is the wordplay.
'plan' indicates anagramming the letters (plan can mean organise).
'resist' anagrammed gives 'SISTER'.
'by' is the link.
(Other definitions for sister that I've seen before include "Nurse, nun or relation" , "Member of family" , "A relative to resist" , "Tender" , "twin?" .)