Pound troy — half-forgotten weight (7)
I believe the answer is:
trounce
'pound' is the definition.
(both can mean to defeat decisively)
'troy half-forgotten weight' is the wordplay.
'half-forgotten' means to take half the letters.
'weight' becomes 'ounce' (ounce is a kind of weight).
'troy' with half the letters taken is 'tr'.
'tr'+'ounce'='TROUNCE'
(Other definitions for trounce that I've seen before include "Easily best" , "Clobber" , "Crush" , "Defeat heavily (in contest)" , "Destroy" .)