Is given nothing but a line on a map (6)
I believe the answer is:
isobar
'a line on a map' is the definition.
(line used in meteorological maps)
'is given nothing but' is the wordplay.
'given' says to put letters next to each other.
'nothing' becomes 'o' (looks like zero - 0).
'but' becomes 'bar' (both can mean 'excluding' or 'except').
'is'+'o'+'bar'='ISOBAR'
(Other definitions for isobar that I've seen before include "Line on map showing barometric pressure" , "curve on map" , "Map line showing atmosphere pressure" , "Pressure line on a weather map" , "Equal-pressure line" .)