Organised party last to seem casual (6)
I believe the answer is:
random
'casual' is the definition.
(without any deliberate order)
'organised party last to seem' is the wordplay.
'organised' becomes 'ran' (I've seen this before).
'party' becomes 'do' (do is a kind of party).
'last to' says to take the final letters.
The last letter of 'seem' is 'm'.
'ran'+'do'+'m'='RANDOM'
(Other definitions for random that I've seen before include "Lacking any definite plan, order or purpose" , "unmethodical" , "Lacking regularity" , "Hit-or-miss" , "Chance, unplanned" .)