Sign of pressure is nothing to pub (6)
I believe the answer is:
isobar
'sign of pressure' is the definition.
(an isobar indicates pressure on a map)
'is nothing to pub' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'to pub' becomes 'bar' (both are drinking establishments).
'is'+'o'+'bar'='ISOBAR'
(Other definitions for isobar that I've seen before include "Line on map showing atmospheric pressure" , "Line that connects places of equal air pressure" , "Line of equal barometric pressure on a map" , "Line connecting points of equal air pressure" , "feature on some maps?" .)