Remains long on eastern island (4)

I believe the answer is:

isle

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'island' is the definition.
(I know that island can be written as isle)

'remains long on eastern' is the wordplay.
'remains long' becomes 'isl' (I am not sure about this - if you are sure you should believe this answer much more).
'on' means one lot of letters go next to another.
'eastern' becomes 'e'.
'isl'+'e'='ISLE'

Can you help me to learn more?

(Other definitions for isle that I've seen before include "Isolated land" , "Of Man or Youth, for example" , "eg Thanet" , "Land encircled by water" , "Eg, Staffa" .)

I've seen this clue in the Sydney Morning Herald.

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