Radiant Rod rocking but slowing down (10)
I believe the answer is:
'slowing down' is the definition.
(I've seen this before)
'radiant rod rocking' is the wordplay.
'rocking' indicates an anagram.
'radiantrod' is an anagram of 'RITARDANDO'.
'but' acts as a link.
(Another definition for ritardando that I've seen is " slowing down".)