Weapon that is a double-edged knife (5)
I believe the answer is:
bowie
'a double-edged knife' is the definition.
'bowie' can be an answer for 'knife' (Bowie knife is an example). I'm not certain of the 'a double-edged' bit.
'weapon that is' is the wordplay.
'weapon' becomes 'bow' (bow is a kind of weapon).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'bow'+'ie'='BOWIE'
Can you help me to learn more?
(Other definitions for bowie that I've seen before include "David --, rock singer/songwriter" , "American frontiersman, killed at the Alamo, 1836" , "Dagger-knife" , "David --, singer" , "Art rocker David" .)