Caught top spy (4)
I believe the answer is:
peek
'spy' is the definition.
(I've seen this before)
'caught top' is the wordplay.
'caught' shows a homophone (sound like) (heard).
'top' becomes 'peak' (synonyms).
'peak' is a homophone of 'PEEK'.
(Other definitions for peek that I've seen before include "Take brief or surreptitious look" , "Glimpse" , "Sly look, peep" , "Quick, sly look" , "Quick or furtive look" .)