I'd heard and observed (4)
I believe the answer is:
eyed
'observed' is the definition.
(I've seen this before)
'i'd heard' is the wordplay.
'heard' indicates a 'sounds like' (homophone) clue.
'id' is a homophone of 'EYED'.
'and' is the link.
(Other definitions for eyed that I've seen before include "Looked at closely" , "Looked at, warily perhaps" , "Watched warily" , "Saw" , "Spotted" .)