Soldier on, steadfastly, though upset when disturbed by grunting sound (5)
I believe the answer is:
snore
'grunting sound' is the definition.
(I've seen this before)
'soldier on steadfastly though upset when disturbed' is the wordplay.
'SNORE' is hidden backwards in amongst the letters.
But, I am unsure how this is indicated.
'by' is the link.
Can you help me to learn more?
(Other definitions for snore that I've seen before include "thing which bores" , "Raucous sound while sleeping" , "How to keep someone awake while sleeping?" , "Make noises sleeping" , "Grunting sound" .)