Organised party membership, initially unsystematic (6)
I believe the answer is:
random
'unsystematic' is the definition.
(thesaurus)
'organised party membership initially' is the wordplay.
'organised' becomes 'ran' (I've seen this before).
'party' becomes 'do' (do is a kind of party).
'initially' says to take the initial letters.
The first letter of 'membership' is 'm'.
'ran'+'do'+'m'='RANDOM'
(Other definitions for random that I've seen before include "Lacking regularity" , "Lacking any definite plan, order or purpose" , "Occurring by chance, no fixed pattern" , "Without pattern" , "Indiscriminate" .)