Prison sentence that is right, first of all (4)
I believe the answer is:
stir
'prison' is the definition.
(informal term for prison)
'sentence that is right first of all' is the wordplay.
'first of all' indicates taking the first letters.
The first letters of 'sentence that is right' is 'STIR'.
(Other definitions for stir that I've seen before include "Begin to move" , "Uproar" , "jail" , "Commotion - prison" , "Disturb" .)