Correct to protect a tree (3)
I believe the answer is:
oak
'tree' is the definition.
(oak is a kind of tree)
'correct to protect a' is the wordplay.
'correct' becomes 'ok'.
'to protect' is an insertion indicator.
'ok' enclosing 'a' is 'OAK'.
(Other definitions for oak that I've seen before include "Strong hardwood" , "Acorns are its fruit" , "Common deciduous tree" , "door at Oxford" , "Tree of the genus Quercus" .)