Look briefly to dispatch good weapon (5)
I believe the answer is:
lance
'weapon' is the definition.
(I know that lance is a type of weapon)
'look briefly to dispatch good' is the wordplay.
'look briefly' becomes 'glance' (I've seen this before).
'to dispatch' indicates named letters should be taken away.
'good' becomes 'g' (abbreviation).
'glance' with 'g' taken away is 'LANCE'.
(Other definitions for lance that I've seen before include "Spear carried by mounted soldier" , "surgical instrument" , "Rider's long weapon" , "steel" , "Jousting weapon" .)