Be an agitator in prison (4)
I believe the answer is:
stir
'prison' is the definition.
(informal term for prison)
'be an agitator in' is the wordplay.
'be an' becomes 'i' (Roman numeral for one).
'agitator' becomes 'str' (I am not sure about this - if you are sure you should believe this answer much more).
'in' means one lot of letters goes inside another.
'i' placed inside 'str' is 'STIR'.
Can you help me to learn more?
(Other definitions for stir that I've seen before include "Prison (sl.); agitate" , "in which you'll find people with convictions" , "Create revolutionary movement" , "Mix the ingredients" , "jail" .)