Resist plan by sibling (6)
I believe the answer is:
sister
'sibling' is the definition.
(I know that sibling can be written as sister)
'resist plan' is the wordplay.
'plan' indicates anagramming the letters (plan can mean organise).
'resist' is an anagram of 'SISTER'.
'by' is the link.
(Other definitions for sister that I've seen before include "A relative to resist" , "Nurse, nun and sibling" , "Resits (anag.)" , "Nurse, nun or relation" , "Martin --, film director" .)