Soldiers annoyed at military error (11,4)
I believe the answer is:
territorial army
'soldiers' is the definition.
(I've seen this before)
'annoyed at military error' is the wordplay.
'annoyed' indicates an anagram.
'at'+'military'+'error'='atmilitaryerror'
'atmilitaryerror' is an anagram of 'TERRITORIAL ARMY'.
(Other definitions for territorial army that I've seen before include "volunteers" , "Soldiers" .)