What’s said to drive away mule? (4)
I believe the answer is:
shoe
'mule?' is the definition.
(mule is a type of shoe)
'what's said to drive away' is the wordplay.
'what's said' indicates a 'sounds like' (homophone) clue.
'to drive away' becomes 'shoo' (shooing is a kind of driving away).
'shoo' is a homophone of 'SHOE'.
(Other definitions for shoe that I've seen before include "Work as a farrier" , "Plate nailed to underside of horse's hoof" , "Slip-on or lace-up?" , "Eg, brogue" , "Flash socket on camera" .)