Not locked up for nothing (4)
I believe the answer is:
free
'for nothing' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for free that I've seen before include "Deliver" , "Without cost; liberate" , "Available - for love" , "Not occupied; release" , "Not fixed; not in use" .)