sleeper
noun
2.
a spy or saboteur or terrorist planted in an enemy country who lives there as a law-abiding citizen until activated by a prearranged signal
3.
an unexpected achiever of success
4.
one of the cross braces that support the rails on a railway track
5.
6.
pajamas with feet; worn by children
7.
8.
tropical fish that resembles a goby and rests quietly on the bottom in shallow water
9.
an unexpected hit