One who legs it if Rob dances badly (9)
I believe the answer is:
absconder
'one who legs it' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'rob dances badly' is the wordplay.
'badly' indicates anagramming the letters (letters in the wrong order).
'rob'+'dances'='robdances'
'robdances' anagrammed gives 'ABSCONDER'.
'if' is the link.
Can you help me to learn more?
(Other definitions for absconder that I've seen before include "Arrest evader" , "Runaway" , "runner" .)