Teach umpire about smell (3)
I believe the answer is:
hum
'smell' is the definition.
(thesaurus)
'teach umpire about' is the wordplay.
'about' says the answer is hidden in the clue.
'HUM' is hidden within 'teach umpire'.
(Other definitions for hum that I've seen before include "Smell strongly" , "Sound (of business?)" , "Buzz; stink" , "Insect sound of doubt" , "Sound of a bee" .)