Boreal throne toppled by sailors (8)
I believe the answer is:
northern
'boreal' is the definition.
(similar in meaning)
'throne toppled by sailors' is the wordplay.
'toppled' indicates anagramming the letters (the order of letters topples or collapses).
'by' means one lot of letters go next to another.
'sailors' becomes 'rn' (abbreviation for Royal Navy).
'throne' is an anagram of 'northe'.
'northe'+'rn'='NORTHERN'
(Other definitions for northern that I've seen before include "14 13" , "On top of the world, map-wise" , "Line represented in black on the Tube map" , "Upwards, as seen in charts" , "London Underground line represented in black" .)