Colour of summit, northern (5)
I believe the answer is:
brown
'colour' is the definition.
(brown is a colour)
'summit northern' is the wordplay.
'summit' becomes 'brow' (brow is a kind of summit).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'brow'+'n'='BROWN'
'of' acts as a link.
(Other definitions for brown that I've seen before include "Dark - tanned - successor to Blair?" , "Colour; ex-PM" , "bookish father" , "Capability" , "Colour of earth" .)