Mob must be mad, losing billions (4)
I believe the answer is:
army
'mob' is the definition.
(thesaurus)
'mad losing billions' is the wordplay.
'mad' becomes 'barmy'.
'losing' is a deletion indicator.
'billions' becomes 'b'.
'barmy' with 'b' taken away is 'ARMY'.
'must be' is the link.
(Other definitions for army that I've seen before include "Large military body" , "Collective term for caterpillars" , "Military unit" , "See 17" , "Fighting force" .)