Messenger, for example, climbing north face of Everest (4)
I believe the answer is:
gene
'messenger' is the definition.
(genes carry genetic information or 'messages')
'example climbing north face of everest' is the wordplay.
'example' becomes 'eg' (short for 'exempli gratia', 'for example' in Latin).
'climbing' is a reversal indicator (in a down clue, letters climb up).
'north' becomes 'N' (abbreviation).
'face of' says to take the initial letters.
The initial letter of 'everest' is 'e'.
'eg' back-to-front is 'ge'.
'ge'+'n'+'e'='GENE'
'for' is the link.
(Other definitions for gene that I've seen before include "Biological factor determining heredity" , "affects someone's make-up" , "Hereditary DNA sequence" , "- - Kelly, dancer" , "-- Pitney, singer" .)