I caught a peeper (3)
I believe the answer is:
eye
'a peeper' is the definition.
(synonyms)
'i caught' is the wordplay.
'caught' shows a homophone (sound like) (to catch something can mean to hear it).
'i' is a homophone of 'EYE'.
(Other definitions for eye that I've seen before include "Sense organ" , "See 1" , "9 across village" , "Light-detector" , "Location of 11dn" .)