Initially is not upset regarding excessive temper (5)
I believe the answer is:
Here is my best explanation:
'excessive temper' is the definition.
The answer and definition can be both related to change as well as being verbs in their base form.
Maybe there's a link between them I don't understand?
'initially is not upset regarding' is the subsidiary indication.
'initially' says to take the initial letters.
'not' becomes 'un' (I've seen this in other clues**).
'upset' says the letters should be written in reverse.
'regarding' becomes 're'.
The initial letter of 'is' is 'i'.
'un' backwards is 'nu'.
(Other definitions for inure that I've seen before include "Harden, make accustomed to", "Become hardened", "get tougher", "Steel", "Accustom - harden", "harden attitude", "Habituate", "Harden against, make tough", "Cause to become hardened to" and "get used?".)