Nonsense — with two wives? (6,5)
I believe the answer is:
'nonsense' is the definition.
(double Dutch is a kind of nonsense)
'with two wives?' is the wordplay.
'two wives?' means two replacements for 'wife'.
'with' says to put letters next to each other.
'wife' becomes 'double' (I am not sure about this - if you are sure you should believe this answer much more).
'wife' becomes 'dutch' (I've seen this before).
'double'+'dutch' is 'DOUBLE-DUTCH'.
(Other definitions for double dutch that I've seen before include "result of bigamy?", "Nonsense (involving two wives?)", "Gibberish (from both Amsterdam and Rotterdam?)", "Incomprehensible language", "Incomprehensible speech".)