Airhead blonde's on fire (6)
I believe the answer is:
alight
'on fire' is the definition.
('alight' can be a synonym of 'on fire')
'airhead blonde's' is the wordplay.
'airhead' becomes 'a' (head letter of 'air').
'blonde' becomes 'light' (I've seen this before).
'a'+'light'='ALIGHT'
(Other definitions for alight that I've seen before include "Get off the bus because it's on fire" , "Descend from vehicle" , "Disembark; on fire" , "Burning; descend" , "'Step off, a bus, say (6)'" .)