Grooved nails (6)
I believe the answer is:
screws
'grooved nails' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for screws that I've seen before include "Fastner with spiral thread" , "Prisoner guards, colloquially" , "Pins with spiral threads" , "Fastening devices and prison guards, colloquially" , "Prison warders (sl.)" .)