Coil on sink is security measure (8)
I believe the answer is:
lockdown
'is security measure' is the definition.
(I've seen this before)
'coil on sink' is the wordplay.
'coil' becomes 'lock'.
'on' says to put letters next to each other (in a down clue, letters appear on others).
'sink' becomes 'down'.
'lock'+'down'='LOCKDOWN'
(Other definitions for lockdown that I've seen before include "Act of confining prisoners to their cells" , "Security measure in prison" .)