Function in garden is occasionally set up (6)
I believe the answer is:
cosine
'function' is the definition.
(cosine is a kind of function)
'in garden is occasionally set up' is the wordplay.
'in' indicates a hidden word.
'set up' says the letters should be written backwards.
'enisoc' can be found hidden inside 'garden is occasionally'.
'enisoc' backwards is 'COSINE'.
(Other definitions for cosine that I've seen before include "Ratio in trigonometry" , "Trig ratio" , "Trigonometric function" , "Trig function" , "Trigonometrical ratio" .)