Weapon, long and narrow, cuts enemies initially (5)
I believe the answer is:
lance
'weapon' is the definition.
(lance is a kind of weapon)
'long and narrow cuts enemies initially' is the wordplay.
'long' becomes 'l'.
'initially' indicates taking the first letters.
The first letters of 'and narrow cuts enemies' is 'ance'.
'l'+'ance'='LANCE'
(Other definitions for lance that I've seen before include "Surgical knife" , "Pointed instrument" , "Incise" , "Long weapon; prick" , "Spear carried by mounted soldier" .)