Sick sense of fun causes irritability (3,6)
I believe the answer is:
ill humour
'fun causes irritability' is the definition.
The definition suggests a singular noun which matches the answer.
'sick sense' is the wordplay.
'sick' becomes 'ill' (similar in meaning).
'sense' becomes 'humour' (I can't justify this - if you can you should believe this answer much more).
'ill'+'humour'='ILL-HUMOUR'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for ill humour that I've seen before include "bad feeling" , "bad mood" , "bad temper" .)