Mood of small son confronting ’umungous lout? (4)
I believe the answer is:
sulk
'mood' is the definition.
(sulk is a kind of mood)
I don't understand the remainder of the clue.
(Other definitions for sulk that I've seen before include "Be bad-tempered and sullen" , "Period of bad temper" , "Be silent and resentful" , "Be sullen and ill-tempered" , "Be in a huff" .)