Briefly made errors performing military instruction (5,4)
I believe the answer is:
order arms
'military instruction' is the definition.
The definition suggests a singular noun which matches the answer.
'briefly made errors performing' is the wordplay.
'briefly' means to remove the last letter.
'performing' is an anagram indicator (the letters move around or 'perform').
'made' with its last letter taken away is 'mad'.
'mad'+'errors'='maderrors'
'maderrors' with letters rearranged gives 'ORDER ARMS'.
Can you help me to learn more?
(Other definitions for order arms that I've seen before include "From stand showing weapons" , "made to officer's requirement" , "Military command" .)