Apprehend a killer at last, then relax (6)
I believe the answer is:
arrest
'apprehend' is the definition.
(I know that apprehend can be written as arrest)
'a killer at last then relax' is the wordplay.
'at last' suggests the final letters.
'then' is a charade indicator (letters next to each other).
'relax' becomes 'rest' (to relax is to have a rest).
The final letter of 'killer' is 'r'.
'a'+'r'+'rest'='ARREST'
(Other definitions for arrest that I've seen before include "Land next to the sea" , "Catch" , "apprehend a law-breaker" , "Stop - detain" , "Seize (a crime suspect)" .)