Move to avoid waltz’s beginning during ball at court (6)
I believe the answer is:
swerve
'move to avoid' is the definition.
'waltz's beginning during ball at court' is the wordplay.
'beginning' suggests taking the first 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 inside 'serve' is 'SWERVE'.
(Other definitions for swerve that I've seen before include "Using spin" , "Sharp turn" , "Veer, turn sharply" , "Change direction abruptly" , "Change course abruptly" .)