Soldiers (6)
'soldiers' is the definition.
(a troop is a group of soldiers)
This is the entire clue.
(Other definitions for troops that I've seen before include "Army" , "Cavalry" , "Military forces" , "Groups of soldiers" , "Soldiers, armed forces" .)
(Other definitions for marines that I've seen before include "Troops trained to serve on land or sea" , "Naval service" , "Seminar for American troops" , "Proverbial recipients of tall stories" , "Soldiers" .)
(Other definitions for menaces that I've seen before include "Threats" , "Threatens" .)