Terrible shortage limiting introduction of powerful ammo once (9)
I believe the answer is:
grapeshot
'ammo once' is the definition.
(historic type of ammunition)
'terrible shortage limiting introduction of powerful' is the wordplay.
'terrible' is an anagram indicator.
'limiting' indicates putting letters inside.
'introduction of' suggests taking the first letters.
The first letter of 'powerful' is 'p'.
'shortage' is an anagram of 'graeshot'.
'graeshot' placed around 'p' is 'GRAPESHOT'.
(Other definitions for grapeshot that I've seen before include "Great Balls of Fire" , "one could get peppered with it!" , "Small iron balls fired from a cannon" , "Charge for barrel" , "Small projectiles fired from cannon" .)