Line on map represents round watering hole (6)
I believe the answer is:
isobar
'line on map' is the definition.
(line indicating pressure on a map)
'represents round watering hole' is the wordplay.
'represents' becomes 'is' (eg a symbol represents or 'is' something else).
'round' becomes 'o' (o is a circular letter).
'watering hole' becomes 'bar' (watering hole can mean a drinking establishment).
'is'+'o'+'bar'='ISOBAR'
(Other definitions for isobar that I've seen before include "Line in front" , "Line joining places of equal barometric pressure" , "Weather-map line" , "Line on map indicating atmospheric pressure" , "It connects points of equal barometric pressure" .)