Unable to say why you can't solve a crossword? (4,3,5)
I believe the answer is:
lost for words
'why you can't solve a crossword?' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'unable to' is the wordplay.
I cannot quite understand how this works, but
'to' could be 'for' and 'for' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'say' acts as a link.
Can you help me to learn more?
(Another definition for lost for words that I've seen is " Speechless - like stumped solver?".)