Hard to say what is unable to swim and fly? (7,4,3,4)

'hard to say what' is the definition.
(something hard to describe)

'is unable to swim and fly?' is the wordplay.
I cannot really see how this works, but
'is' is within the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is found within the answer.
'unable to' could be 'no' (I've seen this in other clues) and 'no' is located in the answer.
This explanation may well be incorrect...