You might attack this grumpy low-down poser (9,6)
I believe the answer is:
crossword puzzle
'poser' is the definition.
(thesaurus)
'you might attack this grumpy low-down' is the wordplay.
I cannot really see how this works, but
'you' could be 'u' (commonly used in online communication) and 'u' is found within the answer.
'this' could be 'crossword' (the crossword currently being solved is an example) and 'crossword' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for crossword puzzle that I've seen is " This".)