A victory in terpsichorean activity is making progress (9)
I believe the answer is:
advancing
'making progress' is the definition.
('advance' can be a synonym of 'make progress')
'a victory in terpsichorean activity' is the wordplay.
'victory' becomes 'v'.
'in' indicates putting letters inside.
'terpsichorean activity' becomes 'dancing' (dancing is a kind of activity. I am not sure about the 'terpsichorean' bit.).
'v' placed into 'dancing' is 'dvancing'.
'a'+'dvancing'='ADVANCING'
'is' is the link.
Can you help me to learn more?
(Other definitions for advancing that I've seen before include "Moving ahead" , "Moving forward like army" .)