Dance truly a breakthrough (9)
I believe the answer is:
discovery
'a breakthrough' is the definition.
(I know that breakthrough can be written as discovery)
'dance truly' is the wordplay.
'dance' becomes 'disco' (disco can mean a dance event).
'truly' becomes 'very' (I've seen this before).
'disco'+'very'='DISCOVERY'
(Other definitions for discovery that I've seen before include "Process of finding" , "old ship" , "Sailing vessel" , "Thing found (or the finding)" , "Detection, unearthing" .)