Enjoys terrifically entertaining Galway festival (6)

I believe the answer is:

oyster

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'galway festival' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'enjoys terrifically entertaining' is the wordplay.
'entertaining' says the answer is hidden in the clue.
'OYSTER' is hidden within 'enjoys terrifically'.

Can you help me to learn more?

(Other definitions for oyster that I've seen before include "Means to cross London" , "aphrodisiac?" , "Tasty bivalve, may carry pearl" , "Succulent bivalve swallowed quickly" , "Bivalve that may nurture a pearl" .)

I've seen this clue in The Irish Times.

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