Celebration not completely unknown? (5)
I believe the answer is:
party
'celebration' is the definition.
(I know that celebration can be written as party)
'not completely unknown?' is the wordplay.
'not completely' becomes 'part' ('part' is the opposite to 'completely').
'unknown?' becomes 'y' (algebra).
'part'+'y'='PARTY'
(Other definitions for party that I've seen before include "Social occasion - political group" , "Social function" , "Shooting weapon" , "festive" , "Political group; function" .)