Not applicable to northern relative (3)
I believe the answer is:
nan
'relative' is the definition.
(I have seen 'Elderly relative' mean 'nan' so perhaps 'relative' could also mean 'nan')
'not applicable to northern' is the wordplay.
'not applicable' becomes 'na' (this might be a standard abbreviation of which I'm unaware).
'to northern' becomes 'n' (abbreviation e.g. N. Ireland).
'na'+'n'='NAN'
Can you help me to learn more?
(Other definitions for nan that I've seen before include "Grandmother (informal)" , "Relation" , "woman from an earlier generation" , "A relative (colloq.); unleavened bread" , "Type of Indian bread" .)