Shoddy ammunition? That's an uncalled-for remark (5,4)
I believe the answer is:
cheap shot
'an uncalled-for remark' is the definition.
The definition suggests a singular noun which matches the answer.
'shoddy ammunition?' is the wordplay.
'shoddy' becomes 'cheap' (I've seen this before).
'ammunition?' becomes 'shot' (I've seen this before).
'cheap'+'shot'='CHEAP SHOT'
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for cheap shot that I've seen before include "Unkind act" , "Barbed comment exploiting known weakness of target" .)