Distinctive stamp long observed in court (6)
I believe the answer is:
cachet
'distinctive stamp' is the definition.
(cachet can mean a stamp or mark)
'long observed in court' is the wordplay.
'long' becomes 'ache' (to long for something is to ache for it).
'observed in' indicates putting letters inside.
'court' becomes 'ct' (abbreviation used in road names).
'ache' going into 'ct' is 'CACHET'.
(Other definitions for cachet that I've seen before include "Kind of prestige, from France" , "Kudos" , "Prestige - seal of approval" , "Seal; mark of prestige" , "Mark of approval" .)