Know instinctively dweller in the far north retains temperature (6)
I believe the answer is:
intuit
'know instinctively' is the definition.
(to know intuitively)
'dweller in the far north retains temperature' is the wordplay.
'dweller in the far north' becomes 'inuit' (the Inuit people live in the far north).
'retains' means one lot of letters goes inside another.
'temperature' becomes 't' (abbreviation).
'inuit' enclosing 't' is 'INTUIT'.
(Other definitions for intuit that I've seen before include "Perform (role) in exaggerated manner" , "Work out by instinct" , "instinctive knowledge" , "Instinctively come to know" , "makes sense" .)