Crop one stored in old prison (5)
I believe the answer is:
maize
'crop' is the definition.
(I've seen this before)
'one stored in old prison' is the wordplay.
'one' becomes 'i' (Roman numeral).
'stored in' is an insertion indicator.
'old prison' becomes 'maze' (Northern Irish prison).
'i' placed within 'maze' is 'MAIZE'.
(Other definitions for maize that I've seen before include "American corn" , "Sweet corn" , "grain crop" , "one could make a meal of it" , "Type of corn" .)