For this reason chicken vacated cage (5)
I believe the answer is:
hence
'for this reason' is the definition.
(thesaurus)
'chicken vacated cage' is the wordplay.
'chicken' becomes 'hen' (hen is a kind of chicken).
'vacated' means to remove the middle letters.
'cage' with its centre taken out is 'ce'.
'hen'+'ce'='HENCE'
(Other definitions for hence that I've seen before include "Therefore, consequently" , "For this reason; in the future" , "As a result" , "From this time, or for this reason" , "As a consequence" .)