Periodically, 54 times nothing mounts up (5)
I believe the answer is:
xenon
'periodically 54 times' is the definition.
I can't judge whether this defines the answer.
'nothing mounts up' is the wordplay.
'nothing' becomes 'none' (I've seen this before).
'mounts' becomes 'x' (I can't explain this - if you can you should believe this answer much more).
'up' is a reversal indicator (in down clue: letters go upwards).
'none'+'x'='nonex'
'nonex' back-to-front is 'XENON'.
Can you help me to learn more?
(Other definitions for xenon that I've seen before include "Chemical element" , "Inert 18" , "One of the noble set" , "Gaseous element, Xe" , "Inert gas used in radio valves" .)