Organised party members, originally haphazard (6)
I believe the answer is:
random
'haphazard' is the definition.
(similar in meaning)
'organised party members originally' is the wordplay.
'organised' becomes 'ran' (I've seen this before).
'party' becomes 'do' (do can informally mean a party).
'originally' indicates taking the first letters.
The first letter of 'members' is 'm'.
'ran'+'do'+'m'='RANDOM'
(Other definitions for random that I've seen before include "Occurring by chance, no fixed pattern" , "Indiscriminate" , "'Haphazard, occurring by chance (6)'" , "without any plan" , "Happening without method" .)