The twin went off with it, the dope! (6)
I believe the answer is:
nitwit
'dope' is the definition.
(I've seen this in another clue)
'the twin went off with it' is the wordplay.
'went off with' is an anagram indicator.
'twin' mixed up with 'it' makes 'NITWIT'.
'the' is the link.
(Other definitions for nitwit that I've seen before include "Blockhead" , "dope" , "Plonker" , "'Foolish, silly person (6)'" , "Turkey" .)