Element 54 (5)
I believe the answer is:
xenon
'element 54' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for xenon that I've seen before include "One of the noble set" , "Heavy, colourless and odourless gas" , "Heavy gaseous element" , "Chemical element" , "Rare gas" .)