Line on map observed at first in island pub (6)
I believe the answer is:
isobar
'line on map' is the definition.
(line indicating pressure on a map)
'observed at first in island pub' is the wordplay.
'at first' says to take the initial letters.
'in' indicates putting letters inside.
'island' becomes 'is'.
'pub' becomes 'bar' (both are drinking establishments).
The initial letter of 'observed' is 'o'.
'is'+'bar'='isbar'
'o' going inside 'isbar' is 'ISOBAR'.
(Other definitions for isobar that I've seen before include "Met map pressure line" , "What's shown on some maps" , "Map line showing atmosphere pressure" , "Evidence of pressure" , "Line on map connecting areas of equal pressure" .)