Private party organised for Rosie at the start of the evening. (6)
I believe the answer is:
soiree
'the evening' is the definition.
(thesaurus)
'private party organised for rosie at the start' is the wordplay.
I cannot quite see how this works, but
an anagram of 'rosie' is 'soire' which is located in the answer.
A single letter 'e' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
'of' is the link.
Can you help me to learn more?
(Other definitions for soiree that I've seen before include "Evening function" , "Cultural at-home" , "Party assembled in the evening" , "A social gathering in the evening" .)