Open up, man! (5)
I believe the answer is:
lance
I believe this clue is a double definition.
'open up' is the first definition.
(lancing is a kind of opening up)
'man' is the second definition.
(Other definitions for lance that I've seen before include "Long pointed weapon of old" , "Cut open" , "Chap" , "Horseman's long weapon" , "corporal may have at the front" .)