Open dispute about Henry's deposition (9)
I believe the answer is:
overthrow
'deposition' is the definition.
(thesaurus)
'open dispute about henry's' is the wordplay.
'open' becomes 'overt' (synonyms).
'dispute' becomes 'row' (row is a kind of dispute).
'about' says to put letters next to each other.
'henry' becomes 'H' (symbol for the unit of electrical inductance).
'row' after 'h' is 'hrow'.
'overt'+'hrow'='OVERTHROW'
(Other definitions for overthrow that I've seen before include "Unseat" , "Bring down rulers" , "chance of another run" , "caused discomfiture" , "Trevor who? (anag)" .)