Northbound part of journey covered by airline for round fare (5)
I believe the answer is:
bagel
'round fare' is the definition.
(I've seen this before)
'northbound part of journey covered by airline' is the wordplay.
'northbound' is a reversal indicator.
'part of journey' becomes 'leg' (leg can mean a stage of a journey).
'covered by' is a charade indicator (letters next to each other) (on top of in a down clue).
'airline' becomes 'BA' (British Airways).
'leg' written backwards gives 'gel'.
'gel' put after 'ba' is 'BAGEL'.
'for' is the link.
(Other definitions for bagel that I've seen before include "Ring-shaped bread product" , "Ring-shaped bread roll" , "Hard bread roll in shape of ring" , "Toroidal bread roll" , "Doughnut-shaped bread roll" .)