Open up to reveal a weapon (5)
I believe the answer is:
lance
This clue is a double definition.
'open up' is the first definition.
(lancing is a kind of opening up)
'a weapon' is the second definition.
(lance is a kind of weapon)
'to reveal' acts as a link.
(Other definitions for lance that I've seen before include "Spear for a horse-soldier" , "Incise" , "Pierce (a boil)" , "Long pointed weapon of old" , "steel" .)