Universal expression of annoyance about singular disturbance (5)
I believe the answer is:
upset
'disturbance' is the definition.
(I know that upset is a type of disturbance)
'universal expression of annoyance about singular' is the wordplay.
'universal' becomes 'U' (film certificate).
'expression of annoyance' becomes 'pet' (pet can mean a bad mood).
'about' indicates putting letters inside.
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'pet' enclosing 's' is 'pset'.
'u'+'pset'='UPSET'
(Other definitions for upset that I've seen before include "Put out" , "Overturn - perturbed - disordered" , "Disturb(ance)" , "Overturn; unhappy" , "unexpectedly beaten?" .)