Reasonable to secure a source of wood (3)
I believe the answer is:
oak
'source of wood' is the definition.
'reasonable to secure a' is the wordplay.
'reasonable' becomes 'ok'.
'to secure' indicates putting letters inside.
'ok' enclosing 'a' is 'OAK'.
(Other definitions for oak that I've seen before include "Traditional British tree" , "Deciduous forest tree" , "Building timber" , "Acorn-bearing tree" , "Nose of certain wines" .)