Multitude, crazy, leaderless (4)
I believe the answer is:
army
'multitude' is the definition.
(I've seen this before)
'crazy leaderless' is the wordplay.
'crazy' becomes 'barmy' (I've seen this before).
'leaderless' means to remove the first letter.
'barmy' with its initial letter removed is 'ARMY'.
(Other definitions for army that I've seen before include "A great number (of soldiers?)" , "Land-fighting force" , "Military land forces" , "service" , "Collective term for caterpillars" .)