Weapon, prototype initially, one many rejected (6)
I believe the answer is:
pistol
'weapon' is the definition.
(pistol is a kind of weapon)
'prototype initially one many rejected' is the wordplay.
'initially' indicates taking the first letters.
'one' becomes 'i' (Roman numeral).
'many' becomes 'lots' (lots of something).
'rejected' says the letters should be written in reverse.
The initial letter of 'prototype' is 'p'.
'lots' reversed gives 'stol'.
'p'+'i'+'stol'='PISTOL'
(Other definitions for pistol that I've seen before include "Spoilt these guns" , "A small hand-gun" , "Hand weapon" , "Small firearm" , "Shakespearean character" .)