Fed up, finish with worker who stands in the dock (9)
I believe the answer is:
defendant
'fed up finish with worker who stands in the dock' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'fed up finish with worker' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'finish' becomes 'end' (synonyms).
'with' means one lot of letters go next to another.
'worker' becomes 'ant' (worker is a type of ant).
'fed' backwards is 'def'.
'def'+'end'+'ant'='DEFENDANT'
Can you help me to learn more?
(Other definitions for defendant that I've seen before include "Person accused" , "Against whom an action is brought in a law court" , "One charged" , "The one before the court" .)