Expression of discontent arrives in sulk before start of holidays (8)
I believe the answer is:
harrumph
'expression of discontent' is the definition.
The definition suggests a verb in its base form which matches the answer.
'arrives in sulk before start of holidays' is the wordplay.
'arrives' becomes 'arr' (I've seen this in other clues).
'in' indicates putting letters inside.
'sulk before' becomes 'hump' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'start of' indicates taking the first letters.
The first letter of 'holidays' is 'h'.
'arr' inserted within 'hump' is 'harrump'.
'harrump'+'h'='HARRUMPH'
Can you help me to learn more?
(Other definitions for harrumph that I've seen before include "Make disapproving sound" , "Clear one's throat noisily" , "Expression of disdain or unhappiness" , "Clearing of the throat - expression of dissatisfaction" , "Noise of throat clearing" .)