Tbsp. oil eradicates/seals unwanted leak (7)

I believe the answer is:

spoiler

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

'unwanted leak' is the definition.
(eg a leak about a film's plot)

'tbsp oil eradicatesseals' is the wordplay.
'SPOILER' is hidden within the letters.
However, I am not sure how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for spoiler that I've seen before include "Disruptive competitor" , "Plot information" , "Sore lip (anag.)" , "attachment to racing car" , "Plot disclosure" .)

I've seen this clue in The Independent.

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