Constituent of panelling in Court Department (4)
I believe the answer is:
wood
'constituent of panelling' is the definition.
(panelling can be made from wood)
'court department' is the wordplay.
'court' becomes 'woo' (synonyms).
'department' becomes 'd' (abbreviation eg in MOD Ministry of Defence).
'woo'+'d'='WOOD'
'in' acts as a link.
(Other definitions for wood that I've seen before include "Area of trees and shrubs" , "Kind of musical instrument" , "Dense growth of trees" , "Grove" , "Large group of trees" .)