The weapon slip, somehow, to inside (6)
I believe the answer is:
pistol
'the weapon' is the definition.
(pistol is a kind of weapon)
'slip somehow to inside' is the wordplay.
'somehow' indicates an anagram.
'inside' means one lot of letters goes inside another.
'slip' with letters rearranged gives 'pisl'.
'pisl' enclosing 'to' is 'PISTOL'.
(Other definitions for pistol that I've seen before include "Arm" , "A small hand-gun" , "making an appearance in Shakespeare's King Henry V" , "Officer's side-arm" , "potential killer?" .)