One's pub has nothing in such a pressure line (6)
I believe the answer is:
isobar
'such a pressure line' is the definition.
(I've seen this before)
'one's pub has nothing' is the wordplay.
'one's' becomes 'is' (roman numeral).
'pub' becomes 'bar' (I've seen this before).
'has' is an insertion indicator.
'nothing' becomes 'o' (looks like zero - 0).
'is'+'bar'='isbar'
'isbar' placed around 'o' is 'ISOBAR'.
'in' acts as a link.
(Other definitions for isobar that I've seen before include "What's shown on some maps" , "Line on map indicating barometric pressure" , "Evidence of pressure" , "Equal-pressure line" , "Contour" .)