End of joke by old king's companion is a gas! (6)
I believe the answer is:
ethane
'gas' is the definition.
(ethane is a gas)
'end of joke by old king's companion' is the wordplay.
'end of' says to take the final letters.
'by' is a charade indicator (letters next to each other).
'old king's companion' becomes 'thane' (historical title for someone given land by a king).
The final letter of 'joke' is 'e'.
'e'+'thane'='ETHANE'
'is a' is the link.
(Other definitions for ethane that I've seen before include "Chemical" , "Compound" , "Athene (anag.); a colourless gas" , "Flammable gas; eat hen (anagram)" , "Flammable hydrocarbon" .)