Trademark: duck behind chunk of wood (4)
I believe the answer is:
logo
'trademark' is the definition.
(logo is a kind of trademark)
'duck behind chunk of wood' is the wordplay.
'duck' becomes 'o'.
'behind' says to put letters next to each other.
'chunk of wood' becomes 'log' (I've seen this before).
'o' put after 'log' is 'LOGO'.
(Other definitions for logo that I've seen before include "Small design used as a company emblem" , "Printed symbol as organisation's emblem" , "Printed symbol as emblem of organisation" , "Emblem, trade-mark" , "(Company) symbol" .)