One function set up; contracted as well for parties (7)
I believe the answer is:
socials
'for parties' is the definition.
(social is a kind of party)
'one function set up contracted as well' is the wordplay.
'one' becomes 'i' (Roman numeral).
'function' becomes 'cos' (mathematical function).
'set up' says the letters should be written backwards (in a down clue, letters go up).
'contracted' means to remove the last letter (to contract is to shorten).
'as well' becomes 'also' (synonyms).
'i'+'cos'='icos'
'icos' in reverse letter order is 'soci'.
'also' with its last letter taken off is 'als'.
'soci'+'als'='SOCIALS'
(Another definition for socials that I've seen is " Parties".)