Wild parties gatecrashed by a freeloader (8)
I believe the answer is:
parasite
'freeloader' is the definition.
(I've seen this in another clue)
'wild parties gatecrashed by a' is the wordplay.
'wild' indicates an anagram.
'gatecrashed by' is an insertion indicator.
'parties' anagrammed gives 'parsite'.
'parsite' going around 'a' is 'PARASITE'.
(Other definitions for parasite that I've seen before include "One living off another" , "sponge" , "Organism living off another" , "Animal that feeds off another" , "one who thrives on (or off) another" .)