King George I, the day before becoming upset (6)
I believe the answer is:
grieve
'upset' is the definition.
(to grieve can mean to cause sorrow)
'king george i the day before' is the wordplay.
'king george i' becomes 'GRI' (Georgius Rex - I is first in Roman numerals).
'the day before' becomes 'eve' (I've seen this before).
'gri'+'eve'='GRIEVE'
'becoming' is the link.
(Other definitions for grieve that I've seen before include "Suffer great sorrow" , "Mourn, lament" , "Suffer intense sorrow" , "Mourn, feel sorrow" , "Cause great distress to" .)