Managed party before mass that's unplanned (6)
I believe the answer is:
random
'unplanned' is the definition.
(I've seen this before)
'managed party before mass' is the wordplay.
'managed' becomes 'ran' (both can mean organised or was in charge of).
'party' becomes 'do' (do is a kind of party).
'before' means one lot of letters go next to another.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'ran'+'do'+'m'='RANDOM'
'that's' acts as a link.
(Other definitions for random that I've seen before include "without any plan" , "Following no method" , "without conscious choice" , "Hit-or-miss" , "spot?" .)