Hold (down) (3)
I believe the answer is:
pin
'hold down' is the definition.
(as in pinning down)
This is all the clue.
(Other definitions for pin that I've seen before include "Fasten; a security code" , "Small object to fasten or attach" , "Peg or nail" , "Skittle; fix" , "Sewer's aid" .)