Modus operandi in case causing bother (6)
I believe the answer is:
molest
'bother' is the definition.
(molesting is a kind of bothering)
'modus operandi in case' is the wordplay.
'modus operandi' becomes 'mo' (common abbreviation).
'in case' becomes 'lest' (I've seen this before).
'mo'+'lest'='MOLEST'
'causing' is the link.
(Other definitions for molest that I've seen before include "Assault in motels" , "Handle roughly, torment" , "Plague" , "badger" , "Attack sexually" .)