Notice electronic agent (4)
I believe the answer is:
espy
'notice' is the definition.
(I've seen this before)
'electronic agent' is the wordplay.
'electronic' becomes 'e' (as in e-mail or e-books).
'agent' becomes 'spy' (spy is a kind of agent).
'e'+'spy'='ESPY'
(Other definitions for espy that I've seen before include "Watch" , "See" , "Detect" , "Catch a glimpse of" , "Perceive" .)