Hardy sergeant's weight (4)
I believe the answer is:
troy
This clue is a double definition.
'hardy sergeant's' is the first definition.
(I've seen this before)
'weight' is the second definition.
(troy is a kind of weight)
(Other definitions for troy that I've seen before include "Helen of --, beauty" , "City destroyed" , "Priam's city" , "Hardy sergeant" , "See 7" .)