They're bitter, also upset about the late finish (5)
I believe the answer is:
aloes
'they're bitter' is the definition.
The definition suggests an adverb but the answer is not.
'also upset about the late finish' is the wordplay.
'upset' is an anagram indicator.
'about' is an insertion indicator.
'finish' suggests the final letters.
The last letter of 'late' is 'e'.
'also' is an anagram of 'alos'.
'alos' placed around 'e' is 'ALOES'.
Can you help me to learn more?
(Another definition for aloes that I've seen is " purgative drug".)