Crossword Genius

Sticker on vessel has cow's head, around which written Farmer's Brew? (8)

Ross

I believe the answer is:

barnacle

I'm a little stuck... Click here to teach me more about this clue!

'sticker on vessel' is the definition.
(a barnacle might stick to a boat)

'cow's head around which written farmer's brew?' is the wordplay.
I cannot quite understand how this works, but
'around' could be 'c' (abbreviation for 'circa') and 'c' is located in the answer.
'brew?' could be 'ale' (ale is a kind of brew) and 'ale' is present in the leftover letters.
The remaining letters 'barn' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

'has' acts as a link.

Can you help me to learn more?

(Other definitions for barnacle that I've seen before include "Crustacean that sticks to rocks etc" , "Clinging crustacean" , "Hull resident (one who clings on)?" , "One that sticks" , "A shellfish that sticks to objects under water" .)

I've seen this clue in The Guardian.
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