Hard things to sharpen knives on? (6)

I believe the answer is:

steels

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'sharpen knives on?' is the definition.
I can't tell whether this defines the answer.

'hard things' is the wordplay.
'hard' becomes 'steel'.
'things' becomes 's' (I can't explain this - if you can you should give a lot more credence to this answer).
'steel'+'s'='STEELS'

'to' is the link.

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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