Look part of castle over (4)
I believe the answer is:
peek
'look' is the definition.
(peek is a kind of look)
'part of castle over' is the wordplay.
'part of castle' becomes 'keep' ('keep' is part of 'castle').
'over' says the letters should be written in reverse.
'keep' in reverse letter order is 'PEEK'.
(Other definitions for peek that I've seen before include "spy" , "Sly look, peep" , "Quick, sly look" , "Furtive glance" , "Glimpse" .)