Feeling one is about to fall (7)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
vertigo
(Other definitions for vertigo that I've seen before include "Type of dizziness associated with fear of heights" , "Dizziness due to disorder in sense of balance" , "Sensitivity to heights" , "Feeling of dizziness generated by heights" , "attack at altitude?" .)