Minimum allowed, while incarcerated (5)
I believe the answer is:
least
'minimum' is the definition.
(similar in meaning)
'allowed while incarcerated' is the wordplay.
'allowed' becomes 'let' (to let is to allow or permit).
'while' becomes 'as' (eg 'while I walked' means 'as I walked').
'incarcerated' means one lot of letters goes inside another.
'let' placed around 'as' is 'LEAST'.
(Other definitions for least that I've seen before include "superlatively small" , "Most slight" , "Bottom" , "Smallest, being stale" , "No more petty" .)