Join or untie badly (5)
I believe the answer is:
unite
'join' is the definition.
(I know that join can be written as unite)
'untie badly' is the wordplay.
'badly' is an anagram indicator (letters in the wrong order).
'untie' anagrammed gives 'UNITE'.
'or' is the link.
(Other definitions for unite that I've seen before include "Get together" , "Put together" , "Join forces" , "pool resources" , "Come together as allies or comrades" .)