Greatly upset dinner partner - forget to go back inside (8)
I believe the answer is:
desolate
'greatly upset' is the definition.
'dinner partner forget to go back inside' is the wordplay.
'dinner partner' becomes 'date'.
'forget' becomes 'lose'.
'to go back' says the letters should be written in reverse.
'inside' means one lot of letters goes inside another.
'lose' backwards is 'esol'.
'date' placed around 'esol' is 'DESOLATE'.
(Other definitions for desolate that I've seen before include "alone" , "Utterly wretched" , "Wretched, unhappy" , "Bereft" , "upset" .)