Old dad ultimately upset one's left home (5)
I believe the answer is:
expat
'one's left home' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'old dad ultimately upset' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'dad' becomes 'pa' (pa can mean someone's father).
'ultimately' says to take the final letters.
The last letter of 'upset' is 't'.
'ex'+'pa'+'t'='EXPAT'
Can you help me to learn more?
(Other definitions for expat that I've seen before include "one settled abroad" , "one working abroad" , "Emigre" , "resident abroad" , "Former countryman" .)