Move easily but risk a tumble (4)
I believe the answer is:
slip
'a tumble' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for slip that I've seen before include "Fail < to engage gear" , "Item of clothing; lose footing" , "Gaffe" , "Mistake; garment" , "Fall on the thinned potter's clay" .)