Attack expression of humour with tons going wrong outside (9)
I believe the answer is:
onslaught
'attack expression' is the definition.
'onslaught' can be an answer for 'attack' (an onslaught is an attack). I am not certain of the 'expression' bit.
'humour with tons going wrong outside' is the wordplay.
'humour with' becomes 'laugh' (laugh is a kind of humour. I am not sure about the 'with' bit.).
'going wrong' indicates anagramming the letters.
'outside' is an insertion indicator.
'tons' is an anagram of 'onst'.
'laugh' placed within 'onst' is 'ONSLAUGHT'.
'of' acts as a link.
(Other definitions for onslaught that I've seen before include "Violent attack" , "offensive action" , "Aggressive action" , "Heavy attack" , "Barrage" .)