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They're seen in cells, tubby policemen eating a bit of shortbread (5,5)

Ross

I believe the answer is:

fatty acids

'they're seen in cells' is the definition.

'tubby policemen eating a bit of shortbread' is the wordplay.
'tubby' becomes 'fatty'.
'policemen' becomes 'cid' (abbreviation for Criminal Investigation Department).
'eating' indicates putting letters inside.
'bit of' suggests taking the first letters.
The first letter of 'shortbread' is 's'.
'fatty'+'cid'='fattycid'
'fattycid' going around 'a' is 'fattyacid'.
'fattyacid'+'s'='FATTY ACIDS'

I've seen this clue in The Everyman puzzle in The Observer.
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