Anger initially infecting army corps (3)
I believe the answer is:
ire
'anger' is the definition.
(ire is intense rage)
'initially infecting army corps' is the wordplay.
'initially' suggests taking the first letters.
'army corps' becomes 'RE' (Royal Engineers).
The initial letter of 'infecting' is 'i'.
'i'+'re'='IRE'
(Other definitions for ire that I've seen before include "Irritation, anger" , "passion" , "Fury" , "Outrage" , "Strong emotion" .)