An organ badly affected by gas (5)
I believe the answer is:
argon
'affected by gas' is the definition.
'argon' can be an answer for 'gas' (argon is a gas). I'm not sure about the 'affected by' bit.
'an organ badly' is the wordplay.
'badly' is an anagram indicator (letters in the wrong order).
'organ' with letters rearranged gives 'ARGON'.
Can you help me to learn more?
(Other definitions for argon that I've seen before include "Noble gas used in light bulbs" , "Gas used in electric lights" , "Gaseous element" , "Inert gas, symbol Ar" , "Inert gas used in light bulbs" .)