Pacific island’s tree-covered area (4)
I believe the answer is:
guam
'pacific island's' is the definition.
(I've seen this before)
'tree-covered area' is the wordplay.
'tree' becomes 'gum' (gum is a kind of tree).
'covered' means one lot of letters goes inside another.
'area' becomes 'a' (maths abbreviation).
'gum' going around 'a' is 'GUAM'.
(Other definitions for guam that I've seen before include "US island east of the Philippines" , "US unincorporated territory in the western Pacific" , "NW Pacific island with large US base" , "North Pacific island, base for the US navy and air force" , "Island in the West Pacific" .)