Observe opening for estate agent (4)
I believe the answer is:
espy
'observe' is the definition.
(I've seen this before)
'opening for estate agent' is the wordplay.
'opening for' says to take the initial 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 "Make out at a distance" , "See" , "Watch" , "Observe, notice" , "Spot, notice" .)