Partygoer in last organised crossing (9)
I believe the answer is:
traversal
'crossing' is the definition.
(I know that traversal is a type of crossing)
'partygoer in last organised' is the wordplay.
'partygoer' becomes 'raver' (I've seen this before).
'in' means one lot of letters goes inside another.
'organised' indicates an anagram.
'last' with letters rearranged gives 'tsal'.
'raver' placed into 'tsal' is 'TRAVERSAL'.
(Other definitions for traversal that I've seen before include "Act of crossing" , "A journey across" , "Action of crossing" , "A crossing (of something)" , "Coming across" .)