Piece of wood securing pole is pine (4)
I believe the answer is:
long
'is pine' is the definition.
(I've seen this before)
'piece of wood securing pole' is the wordplay.
'piece of wood' becomes 'log'.
'securing' means one lot of letters goes inside another.
'pole' becomes 'n'.
'log' enclosing 'n' is 'LONG'.
(Other definitions for long that I've seen before include "dream" , "Really want" , "to be tall" , "summer break?" , "Lengthy - pine" .)