Move to avoid waltz’s beginning during ball at court (6)
I believe the answer is:
Here is my best explanation:
'move to avoid' is the definition.
'waltz's beginning during ball at court' is the subsidiary indication.
'beginning' says to take the initial letters.
'during' is an insertion indicator.
'ball at court' becomes 'serve' (term on a tennis court).
The first letter of 'waltzs' is 'w'.
'w' inserted into 'serve' is 'swerve'.
(Other definitions for swerve that I've seen before include "Veer, turn sharply", "Change course suddenly", "Change course violently", "Suddenly move off course", "Dodge", "Sharp turn", "Sudden deviation from straight course", "Move suddenly off course", "try to avoid" and "Abrupt change of course".)