Weapon common round Latvia's capital (5)
I believe the answer is:
rifle
'weapon' is the definition.
(rifle is a kind of weapon)
'common round latvia's capital' is the wordplay.
'common' becomes 'rife' (I can't justify this - if you can you should believe this answer much more).
'round' is an insertion indicator.
'capital' suggests taking the first letters.
The first letter of 'latvia' is 'l'.
'rife' enclosing 'l' is 'RIFLE'.
Can you help me to learn more?
(Other definitions for rifle that I've seen before include "Firearm - plunder" , "Flier (anag.)" , "Gun - ransack" , "Carbine" , "Winchester" .)