Niece isn't struggling with Gran's stubbornness (13)
I believe the answer is:
intransigence
'stubbornness' is the definition.
(I know that intransigence is a type of stubbornness)
'niece isn't struggling with gran's' is the wordplay.
'struggling with' indicates an anagram (the letters struggle or wriggle into a new form).
'niece'+'isnt'='nieceisnt'
'nieceisnt' mixed up with 'gran' makes 'INTRANSIGENCE'.
(Other definitions for intransigence that I've seen before include "Stubbornness" , "Refusal to change one's opinions" , "Unwillingness to agree" , "uncompromising attitude" , "pigheadedness" .)