Piece of security (4)
I believe the answer is:
pawn
I believe this is a double definition.
'piece' is the first definition.
(I know that pawn is a type of chess piece)
'security' is the second definition.
(I've seen this before)
'of' acts as a link.
(Other definitions for pawn that I've seen before include "Man of lowest value" , "Minor chessman" , "Chess piece; hock" , "To pop, hock" , "Chess piece -- puppet" .)