Secret look back to find castle (4)
I believe the answer is:
keep
'castle' is the definition.
(keep is a type of castle)
'secret look back' is the wordplay.
'secret look' becomes 'peek' (I've seen this in another clue).
'back' says the letters should be written in reverse.
'peek' in reverse letter order is 'KEEP'.
'to find' is the link.
(Other definitions for keep that I've seen before include "Observe - reserve - preserve" , "Castle tower; cost of maintenance" , "Stock" , "Conserve" , "Save" .)