Leading estate agent's notice (4)
I believe the answer is:
espy
'notice' is the definition.
(thesaurus)
'leading estate agent's' is the wordplay.
'leading' indicates taking the first letters.
'agent' becomes 'spy' (spy is a kind of agent).
The initial letter of 'estate' is 'e'.
'e'+'spy'='ESPY'
(Other definitions for espy that I've seen before include "Watch" , "Spot; observe" , "Make out at a distance" , "Perceive" , "Discern" .)