Deputise for one at court (3)
I believe the answer is:
act
'court' is the definition.
(I know that court is a more specific form of the action act)
'deputise for one' is the wordplay.
'deputise' becomes 'ct' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'for' is a charade indicator (letters next to each other) (I've seen this in other clues).
'one' becomes 'a' (a thing is one thing).
'ct' put after 'a' is 'ACT'.
'at' acts as a link.
Can you help me to learn more?
(Other definitions for act that I've seen before include "Do something, in a play?" , "work" , "Parliamentary decree" , "Take part in a play" , "Major division of play" .)