Buffoon's laughter conceals fierce attack (9)
I believe the answer is:
onslaught
'fierce attack' is the definition.
(I've seen this before)
'buffoon's laughter conceals' is the wordplay.
'conceals' indicates a hidden word.
'ONSLAUGHT' is hidden in the letters of 'buffoons laughter'.
(Other definitions for onslaught that I've seen before include "Aggressive action" , "Sally" , "Vicious attack" , "Heavy attack" , "Fierce attack" .)