Put it in writing that it's hardened, though it feels soft (7)

'it's hardened though it feels soft' is the definition.
I know nothing about this answer so I can't tell whether this works.

'put it in writing' is the wordplay.
I cannot really see how this works, but
'put' could be 'set' (both can mean to place or position) and 'set' is found within the answer.
'in writing' could be 'down' (as in written down) and 'down' is present in the answer.
No letters remain.
This explanation may well be incorrect...

'that' is the link.

Can you help me to learn more?