Organised party meetings, originally irregular (6)
I believe the answer is:
'irregular' is the definition.
(I know that irregular can be written as random)
'organised party meetings 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 first letter of 'meetings' is 'm'.
(Other definitions for random that I've seen before include "Lacking any definite plan, order or purpose" , "Happening without method" , "'Haphazard, occurring by chance (6)'" , "Arbitrary" , "casual?" .)