Detain under suspicion (6)
I believe the answer is:
arrest
'detain under suspicion' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for arrest that I've seen before include "Catch" , "Land next to the sea" , "Stop (on suspicion)" , "Sartre (anag) -- check" , "Cause to stop" .)