Element of attack happening by air (5)
I believe the answer is:
radon
'element of attack' is the definition.
'radon' can be an answer for 'element' (radon is a kind of element). I am not certain of the 'of attack' bit.
'happening by air' is the wordplay.
'happening' becomes 'on'.
'by' means one lot of letters go next to another.
'air' becomes 'rad' (I can't explain this - if you can you should believe this answer much more).
'on' after 'rad' is 'RADON'.
Can you help me to learn more?
(Other definitions for radon that I've seen before include "Inert gas - adorn (anag)" , "Natural gas considered a hazard to health" , "Gaseous element, Rn" , "A radioactive gas, symbol Rn" , "Element, symbol Rn" .)