Tree unknown on northeastern island (8)
I believe the answer is:
alderney
'island' is the definition.
(I've seen this before)
'tree unknown on northeastern' is the wordplay.
'tree' becomes 'alder' (alder tree is a kind of tree).
'unknown' becomes 'y' (algebra).
'on' means one lot of letters go next to another.
'northeastern' becomes 'ne' (abbreviation for northeastern).
'y' after 'ne' is 'ney'.
'alder'+'ney'='ALDERNEY'
(Other definitions for alderney that I've seen before include "Cattle" , "A Channel Island" , "Cow, perhaps" , "One of Channel Islands" , "Third largest of the Channel Islands" .)