Public line about Henry's defeat (9)
I believe the answer is:
overthrow
'defeat' is the definition.
(I've seen this before)
'public line about henry's' is the wordplay.
'public' becomes 'overt' ('overt' is associated in meaning with 'public').
'line' becomes 'row' (row is a kind of line**).
'about' says to put letters next to each other.
'henry' becomes 'h' ('H' can be a synonym of 'henry').
'row' after 'h' is 'hrow'.
'overt'+'hrow'='OVERTHROW'
(Other definitions for overthrow that I've seen before include "Remove forcibly from power" , "Trevor who? (anag)" , "Extra run perhaps" , "Unseat" , "Bring down rulers" .)