Peeper (3)
I believe the answer is:
eye
'peeper' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for eye that I've seen before include "'Public' or 'private' body part" , "1 across, 4 across, for example" , "Organ of sight" , "20 [OBSERVE]" , "Look at with interest" .)