Johnson's laughter seen partly as an attack (9)
I believe the answer is:
onslaught
'an attack' is the definition.
('onslaught' can be a synonym of 'attack')
'johnson's laughter seen partly' is the wordplay.
'partly' indicates a hidden word.
'ONSLAUGHT' is hidden within 'johnsons laughter seen'.
'as' acts as a link.
(Other definitions for onslaught that I've seen before include "Destructive attack" , "Heavy attack" , "Violent attack" , "Fierce attack" , "Barrage" .)