Tertiary function of some insurance (5,5)
I believe the answer is:
third party
'some insurance' is the definition.
(I've seen this before)
'tertiary function' is the wordplay.
'tertiary' becomes 'third' (synonyms).
'function' becomes 'party' (function can mean a social event).
'third'+'party'='THIRD-PARTY'
'of' acts as a link.
(Other definitions for third party that I've seen before include "Liberal Democrats?" , "Another (uninvolved) person" , "Person not directly involved" , "someone injured by driver?" , "Liberals once?" .)