Picked out a unique hardback! (7)

I believe the answer is:

singled

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'picked' is the definition.
The definition and answer can be both to do with contact as well as being past participle verbs.
Maybe they are linked in a way I don't understand?

'a unique hardback' is the wordplay.
'a unique' becomes 'single' (I've seen this before).
'hardback' becomes 'd' (I can't explain this - if you can you should give a lot more credence to this answer).
'single'+'d'='SINGLED'

'out' is the link.

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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