Hold the murderer, not tied up, inside (7)
I believe the answer is:
contain
'hold' is the definition.
(synonyms)
'the murderer not tied up inside' is the wordplay.
'the murderer' becomes 'cain' (Cain who killed Abel).
'not tied up' becomes 'ont' (I can't explain this - if you can you should believe this answer much more).
'inside' is an insertion indicator.
'cain' going around 'ont' is 'CONTAIN'.
Can you help me to learn more?
(Other definitions for contain that I've seen before include "Accommodate" , "I cannot (anag) - hold back" , "Carry" , "Keep within limits or have within" , "keep within bounds" .)