Hold the murderer, not tied up, inside (7)
I believe the answer is:
contain
'hold' is the definition.
(synonyms)
'murderer not tied up inside' is the wordplay.
'murderer' becomes 'cain' (Cain who killed Abel).
'tied up' indicates anagramming the letters (letters get tied or tangled up).
'inside' indicates putting letters inside.
'not' with letters rearranged gives 'ont'.
'cain' placed around 'ont' is 'CONTAIN'.
'the' acts as a link.
(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" .)