Could you sing to him when dancing? (3)
I believe the answer is:
dan
'could you sing to him when dancing?' is the definition.
The definition suggests a singular noun which matches the answer.
This is all the clue.
Can you help me to learn more?
(Other definitions for dan that I've seen before include "Judo level of proficiency (3)" , "Jacob's fourth son" , "Proficient combative sportsman" , "Judo level and Wicklow lake" , "-- Dare, comic hero" .)