Originally serving time in Reading jail (4)
I believe the answer is:
stir
'jail' is the definition.
(stir can informally mean jail)
'originally serving time in reading' is the wordplay.
'originally' says to take the initial letters.
The first letters of 'serving time in reading' is 'STIR'.
(Other definitions for stir that I've seen before include "Mix the ingredients" , "A slangy prison - whisk it" , "Slight movement" , "Move; wake" , "Whisk or mix" .)