Favourite weapon to contain universal ill-humour (9)
I believe the answer is:
petulance
'ill-humour' is the definition.
'favourite weapon to contain universal' is the wordplay.
'favourite' becomes 'pet' (synonyms).
'weapon' becomes 'lance' (lance is a kind of weapon).
'to contain' indicates putting letters inside.
'universal' becomes 'u'.
'pet'+'lance'='petlance'
'petlance' going around 'u' is 'PETULANCE'.
(Other definitions for petulance that I've seen before include "Peevish irritation" , "Peevishness" , "Childish sulkiness" , "Childish bad-temper" , "Sulky bad behaviour" .)