Trademark is piece of wood with circle (4)
I believe the answer is:
logo
'trademark' is the definition.
(logo is a kind of trademark)
'piece of wood with circle' is the wordplay.
'piece of wood' becomes 'log'.
'with' is a charade indicator (letters next to each other).
'circle' becomes 'o'.
'log'+'o'='LOGO'
'is' is the link.
(Other definitions for logo that I've seen before include "A company's printed symbol or emblem" , "Printed symbol as emblem of organisation" , "Emblematic design" , "Emblem, trade-mark" , "Symbol or emblem used by organisation" .)