Pressure line from one's pub carrying oxygen (6)
I believe the answer is:
isobar
'pressure line' is the definition.
(I've seen this before)
'one's pub carrying oxygen' is the wordplay.
'one's' becomes 'is' (roman numeral).
'pub' becomes 'bar' (I've seen this before).
'carrying' says to put letters next to each other.
'oxygen' becomes 'o' (atomic symbol for oxygen).
'bar' after 'o' is 'obar'.
'is'+'obar'='ISOBAR'
'from' acts as a link.
(Other definitions for isobar that I've seen before include "One showing pressure" , "sign of pressure" , "Line on map showing atmospheric pressure" , "Line on a map connecting points of equal atmospheric pressure" , "Line on map showing barometric pressure" .)