They're bitter, also upset about the late finish (5)

(Source: London Evening Standard)

"also upset" ⇒ anagram of "also" ⇒ e.g. "alos"

"late finish" ⇒ last letter or letters of "late" ⇒ "e"

"alos" about "e" ⇒ the letter "e" inserted into the letters "alos"

An example of something bitter

"aloes"