Plant line across Eastern Pacific (9)
I believe the answer is:
peaceable
'pacific' is the definition.
('peaceable' can be a synonym of 'pacific')
'plant line across eastern' is the wordplay.
'plant' becomes 'pea'.
'line' becomes 'cable' (cable is a kind of line).
'across' means one lot of letters goes inside another.
'eastern' becomes 'e'.
'cable' going around 'e' is 'ceable'.
'pea'+'ceable'='PEACEABLE'
(Other definitions for peaceable that I've seen before include "not aggressive" , "Unwarlike" .)