Hostile to North, this person’s to scheme underhandedly (7)
I believe the answer is:
connive
'scheme underhandedly' is the definition.
'hostile to north this person's' is the wordplay.
'hostile to' becomes 'con' (con can mean opposed to).
'north' becomes 'n' ('N' can be a synonym of 'north').
'this person's' becomes 'ive' (as in 'i've', referring to the crossword setter).
'con'+'n'+'ive'='CONNIVE'
'to' is the link.
(Other definitions for connive that I've seen before include "Plot, conspire" , "Intrigue, scheme" , "Conspire; wink (at 2)" , "Plot secretly" , "Conspire (to do something harmful)" .)