Function turns out so nice (6)

I believe the answer is:

cosine

'function' is the definition.
(I know that cosine is a type of circular function)

'turns out so nice' is the wordplay.
'turns out' indicates an anagram.
'so'+'nice'='sonice'
'sonice' anagrammed gives 'COSINE'.

(Other definitions for cosine that I've seen before include "Trig function" , "Trigonometric function" , "Ratio in trigonometry" , "Trigonometrical ratio" , "Trig ratio" .)

I've seen this clue in The Mirror.

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