Not good imprisoning 50 without locks (4)
I believe the answer is:
bald
'without locks' is the definition.
(without locks of hair)
'not good imprisoning 50' is the wordplay.
'not good' becomes 'bad' (bad is the opposite of good).
'imprisoning' is an insertion indicator.
'50' becomes 'l' (Roman numeral).
'bad' placed around 'l' is 'BALD'.
(Other definitions for bald that I've seen before include "Lacking hair on the head" , "Unadorned" , "Lacking hair on the scalp" , "Not covered with hair" , "Without the natural covering" .)