Same pressure line is on top counter (6)
I believe the answer is:
isobar
'same pressure line' is the definition.
'isobar' can be an answer for 'line' (isobar is a kind of line). I'm not sure about the 'same pressure' bit.
'is on top counter' is the wordplay.
'top' indicates taking the first letters (in a down clue, the first letter is top).
'counter' becomes 'bar' (bar is a kind of counter).
The first letter of 'on' is 'o'.
'is'+'o'+'bar'='ISOBAR'
Can you help me to learn more?
(Other definitions for isobar that I've seen before include "(Map) line of constant atmospheric pressure" , "meteorologist's line" , "Line on map connecting places of equal atmospheric pressure" , "Line joining places of equal barometric pressure" , "feature on some maps?" .)