Everything that is bad finally is related (6)
I believe the answer is:
allied
'is related' is the definition.
(similar in meaning)
'everything that is bad finally' is the wordplay.
'everything' becomes 'all' (all things).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'finally' suggests the final letters.
The last letter of 'bad' is 'd'.
'all'+'ie'+'d'='ALLIED'
(Other definitions for allied that I've seen before include "(Of nations) working together" , "Affiliated" , "Joined together for a purpose" , "Owl's cry" , "Joined by, eg, treaty" .)