Line on certain maps represents old pub (6)
I believe the answer is:
isobar
'line on certain maps' is the definition.
'isobar' can be an answer for 'line' (I've seen this before). I'm not sure about the 'on certain maps' bit.
'represents old pub' is the wordplay.
'represents' becomes 'is' (eg a symbol represents or 'is' something else).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'pub' becomes 'bar' (both are drinking establishments).
'is'+'o'+'bar'='ISOBAR'
Can you help me to learn more?
(Other definitions for isobar that I've seen before include "Line connecting points of equal air pressure" , "Line that connects places of equal air pressure" , "Line on map indicating barometric pressure" , "What's shown on some maps" , "Map line showing atmosphere pressure" .)