Middle of Blackadder set ruined (5)
I believe the answer is:
kaput
'ruined' is the definition.
(I've seen this before)
'middle of blackadder set' is the wordplay.
'middle of' means to look at the middle letters.
'set' becomes 'put' (synonyms).
The middle of 'blackadder' is 'ka'.
'ka'+'put'='KAPUT'
(Other definitions for kaput that I've seen before include "Permanently broken" , "Broken or finished - in Germany" , "not 10 [IN ORDER]" , "Slangily broken, from German" , "Completely broken down" .)