To-do caused by us in force, southern (4)
I believe the answer is:
fuss
'to-do' is the definition.
(I've seen this before)
'us in force southern' is the wordplay.
'in' indicates putting letters inside.
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'southern' becomes 's' (abbreviation).
'f'+'s'='fs'
'us' placed inside 'fs' is 'FUSS'.
'caused by' is the link.
(Other definitions for fuss that I've seen before include "Bustle; ado" , "Commotion; ado" , "Display of unnecessary interest" , "Bustle; commotion" , "Bustle and bother" .)