Side with me mistakenly in the end (6)
I believe the answer is:
demise
'the end' is the definition.
(demise is a kind of end)
'side with me mistakenly' is the wordplay.
'with' means one lot of letters go next to another.
'mistakenly' is an anagram indicator.
'side'+'me'='sideme'
'sideme' is an anagram of 'DEMISE'.
'in' is the link.
(Other definitions for demise that I've seen before include "The end of something" , "Death, collapse" , "Death or expiration" , "Passing" , "(Person's) death" .)