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" .)