Setter supports appeal over jail sentence (4)
I believe the answer is:
time
'jail sentence' is the definition.
'time' can be an answer for 'sentence' (a prison sentence). I am not sure about the 'jail' bit.
'setter supports appeal over' is the wordplay.
'setter' becomes 'me' (the setter of the clue).
'supports' means one lot of letters go next to another (bottom letters provide a support for the top ones).
'appeal' becomes 'it' ('it' can mean something attractive or appealing).
'over' says the letters should be written in reverse.
'it' written backwards gives 'ti'.
'me' put after 'ti' is 'TIME'.
Can you help me to learn more?
(Other definitions for time that I've seen before include "Continuum of past, present and future" , "Prison sentence; occasion" , "Speech to close pub" , "Procrastination is the thief of ." , ""The enemy"" .)