Set up in advance, social function (5)
I believe the answer is:
cosec
'function' is the definition.
(mathematical function)
'set up in advance social' is the wordplay.
'set up' says the letters should be written backwards (in a down clue, letters go up).
'in' indicates the answer is hidden within the clue.
'cesoc' is hidden in the letters of 'advance social'.
'cesoc' backwards is 'COSEC'.
(Other definitions for cosec that I've seen before include "(In maths) reciprocal of sine (abbr)" , "(In mathematics) the reciprocal of sine (abbr)" , "Inverse of sine (abbr.)" , "company secretary" , "Trig function" .)