Resists nasty relations (7)
I believe the answer is:
sisters
'relations' is the definition.
(I know this)
'resists nasty' is the wordplay.
'nasty' indicates anagramming the letters (a nasty or wrong spelling).
'resists' is an anagram of 'SISTERS'.
(Other definitions for sisters that I've seen before include "Resists (anag)" , "Members of the family" , "Family relatives" , "Nuns - nurses - siblings" , "Relatives in hospital?" .)