It’s supporting the Dog and Duck perhaps, is that not so? (5)
I believe the answer is:
innit
'is that not so?' is the definition.
(I've seen this before)
'it's supporting the dog and' is the wordplay.
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'the dog' becomes 'in' (I can't explain this - if you can you should give a lot more credence to this answer).
'and' becomes 'n' (common abbreviation for 'and').
'in'+'n'='inn'
'it' after 'inn' is 'INNIT'.
'duck perhaps' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for innit that I've seen before include "word of emphasis" , "Isn't it (sl.)" , "tag question" , "that produces stress for some" , "Mere spoken punctuation" .)