Apprehend a wrongdoer 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 wrongdoer at last then relax' is the wordplay.
'at last' suggests the final letters.
'then' says to put letters next to each other.
'relax' becomes 'rest' (to relax is to have a rest).
The last letter of 'wrongdoer' is 'r'.
'a'+'r'+'rest'='ARREST'
(Other definitions for arrest that I've seen before include "Take into custody as police do" , "Catch" , "Stop (on suspicion)" , "Nab" , "Stop - detain" .)