Included in holidays in Cork (3)
I believe the answer is:
lid
'cork' is the definition.
(thesaurus)
'included in holidays in' is the wordplay.
'included in' indicates a hidden word.
'LID' is hidden within 'holidays in'.
(Other definitions for lid that I've seen before include "Kind of cover, may be hinged" , "Cover from gliders" , "Receptacle cover" , "Removable cover of a container" , "Top or cover" .)