Old people seem upset about the sad ending (5)

'old people' is the definition.
(people of the Median Empire)

'seem upset about the sad ending' is the wordplay.
'upset' says the letters should be written backwards.
'about' means one lot of letters goes inside another.
'ending' says to take the final letters (the ending of the word).
The final letter of 'sad' is 'd'.
'seem' written backwards gives 'mees'.
'mees' placed around 'd' is 'MEDES'.