Observer I picked up (3)
I believe the answer is:
eye
'observer' is the definition.
(organ which sees or observes)
'i picked up' is the wordplay.
'picked up' indicates a 'sounds like' (homophone) clue.
'i' is a homophone of 'EYE'.
(Other definitions for eye that I've seen before include "One looks" , "Location of 11dn" , "Good discernment" , "Look at; centre" , "Globular organ" .)