Area covered in fine wood (3)
I believe the answer is:
oak
'wood' is the definition.
(I know that oak is a type of wood)
'area covered in fine' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'covered in' is an insertion indicator.
'fine' becomes 'OK' (synonyms).
'a' placed within 'ok' is 'OAK'.
(Other definitions for oak that I've seen before include "Acorn tree" , "Large tree" , "Strong hardwood" , "Common forest tree" , "Its seed is the acorn" .)