Crossword Genius

… to then cover boat’s prow in varnish? (4)

Ross

I believe the answer is:

coat

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

'varnish?' is the definition.
(I know that varnish is a more specific form of coat)

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for coat that I've seen before include "Outer covering" , "Layer of paint" , "Jacket" , "Overgarment" , "blanket" .)

I've seen this clue in The Independent.
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