Protein book now available at bookshop? (7)

I believe the answer is:

albumin

'protein' is the definition.
(albumin is a kind of protein)

'book now available at bookshop?' is the wordplay.
'book' becomes 'album' (album is a kind of book).
'now available at bookshop?' becomes 'in' (I can't explain this - if you can you should give a lot more credence to this answer).
'album'+'in'='ALBUMIN'

(Another definition for albumin that I've seen is " Protein".)

I've seen this clue in The Times.

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