Mistakenly untied, so joined together (6)
I believe the answer is:
united
'joined together' is the definition.
(I've seen this in another clue)
'mistakenly untied' is the wordplay.
'mistakenly' is an anagram indicator.
'untied' with letters rearranged gives 'UNITED'.
'so' acts as a link.
(Other definitions for united that I've seen before include "... we stand, divided we fall" , "See 14" , "Joined together as one" , "in league" , "Joined or combined" .)