Line on the map from one returning overseas to tavern (6)
I believe the answer is:
isobar
'line on the map' is the definition.
(I've seen this before)
'one returning overseas to tavern' is the wordplay.
'one' becomes 'i' (Roman numeral).
'returning' says the letters should be written in reverse.
'overseas' becomes 'os' (abbreviation).
'to tavern' becomes 'bar' (I've seen this in another clue).
'os' in reverse letter order is 'so'.
'i'+'so'+'bar'='ISOBAR'
'from' acts as a link.
(Other definitions for isobar that I've seen before include "Equal-pressure line" , "Line joining places of equal barometric pressure" , "Line showing points having equal barometric pressure at a given time" , "Barometric curve" , "Air pressure line on a weather chart" .)