Would it damage part of a shoe? It’s hard to say (6-7)
I believe the answer is:
tongue-twister
'it's hard to say' is the definition.
(I've seen this in another clue)
'would it damage part of a shoe?' is the wordplay.
I cannot really see how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found within the answer.
'part' could be 'te' (te is a kind of part) and 'te' is found in the answer.
'of' could be 'o' and 'o' is located in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for tongue-twister that I've seen before include "Peter Piper picked a peck of pickled pepper, for instance" , "Sequence of words difficult to say" , ""I wish to wash my Irish wristwatch", perhaps" , ""Red lorry, yellow lorry", for example" , "Phrase that is difficult to say properly" .)