Initially is not upset regarding excessive temper (5)
I believe the answer is:
inure
'temper' is the definition.
(I've seen this before)
'initially is not upset regarding excessive' is the wordplay.
'initially' says to take the initial letters.
The first letters of 'is not upset regarding excessive' is 'INURE'.
(Other definitions for inure that I've seen before include "Harden against, make tough" , "Harden or season" , "Harden, make accustomed to" , "Temper, toughen" , "harden attitude" .)