Organised party machine, originally unsystematic (6)
I believe the answer is:
random
'unsystematic' is the definition.
(thesaurus)
'organised party machine originally' is the wordplay.
'organised' becomes 'ran' (I've seen this before).
'party' becomes 'do' (do is a kind of party).
'originally' suggests taking the first letters.
The initial letter of 'machine' is 'm'.
'ran'+'do'+'m'='RANDOM'
(Other definitions for random that I've seen before include "Wild" , "'Haphazard, occurring by chance (6)'" , "Without pattern" , "Haphazard, no fixed pattern" , "Without a particular aim, haphazard as choice may be" .)