A twin Hollander? What rubbish! (6,5)
I believe the answer is:
double dutch
'what rubbish' is the definition.
I can't tell whether this definition defines the answer.
'a twin hollander?' is the wordplay.
'a twin' becomes 'double' (identical twins are each other's doubles).
'hollander?' becomes 'dutch' (I can't explain this - if you can you should believe this answer much more).
'double'+'dutch'='DOUBLE-DUTCH'
Can you help me to learn more?
(Other definitions for double dutch that I've seen before include "Nonsensical" , "Incomprehensible language" , "Incomprehensible speech" , "Gibberish (from both Amsterdam and Rotterdam?)" , "result of bigamy?" .)