In a film, one in seven is so cheerful (5)

I believe the answer is:

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'cheerful' is the definition.
(associated in meaning)

I cannot understand how the rest of the clue works.

(Other definitions for happy that I've seen before include "Well-pleased" , "A not so low" , "Pleased; fortunate" , "Doc's smiling pal" , "One of the Seven Dwarves" .)

I've seen this clue in the Evening Standard.

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