Harry beating Henry guaranteed (2,3,3)
I believe the answer is:
in the bag
'guaranteed' is the definition.
(I've seen this in another clue)
'harry beating henry' is the wordplay.
'harry' indicates an anagram (harry can mean to attack or spoil).
'henry' becomes 'H' (symbol for the unit of electrical inductance).
'beating'+'h'='beatingh'
'beatingh' anagrammed gives 'IN THE BAG'.
(Other definitions for in the bag that I've seen before include "Where mail may be" , "As good as done" , "Virtually secured (informal)" , "Almost assured of succeeding" , "Informally secured as one wished" .)