Some upset escapee kidnapped from castle (4)
I believe the answer is:
keep
'castle' is the definition.
(thesaurus)
'some upset escapee kidnapped' is the wordplay.
'upset' is a reversal indicator.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'peek' can be found hidden inside 'escapee kidnapped'.
'peek' in reverse letter order is 'KEEP'.
'from' is the link.
(Other definitions for keep that I've seen before include "Main tower of a medieval castle" , "Act as custodian of" , "Conserve" , "Board and lodging" , "Hold - fortress" .)