Weapon has first round in place (6)
I believe the answer is:
pistol
'weapon' is the definition.
(pistol is a kind of weapon)
'has first round in place' is the wordplay.
'has first' becomes 'ist' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'round' becomes 'o' (round can mean a round object).
'in' means one lot of letters goes inside another.
'place' becomes 'pl' (I've seen this in other clues).
'ist'+'o'='isto'
'isto' inserted inside 'pl' is 'PISTOL'.
(Other definitions for pistol that I've seen before include "Small firearm" , "making an appearance in Shakespeare's King Henry V" , "Piece" , "Spoilt these guns" , "A small hand-gun" .)