Secretly look up to find castle tower (4)
I believe the answer is:
keep
'castle tower' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for keep that I've seen before include "Hold - part of castle" , "honour" , "Look after" , "Only one cricketer on field will" , "Retain; strong tower" .)