What ballerinas dance on (6)
I believe the answer is:
tiptoe
'what ballerinas dance on' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tiptoe that I've seen before include "Go lightly and stealthily" , "Walk very quietly with heels raised" , "Move delicately" , "''... through the tulips'' (Tiny Tim song)" , "Move stealthily, with heels raised" .)