Open file clinching Harmon's first defeat (9)
I believe the answer is:
overthrow
'defeat' is the definition.
(I've seen this before)
'open file clinching harmon's first' is the wordplay.
'open' becomes 'overt' (synonyms).
'file' becomes 'row' (a file is a row of squares e.g. on a chess board).
'clinching' is an insertion indicator.
'harmon's first' becomes 'h' (ost letter of 'harmon').
'overt'+'row'='overtrow'
'overtrow' enclosing 'h' is 'OVERTHROW'.
(Other definitions for overthrow that I've seen before include "Bring extra" , "Remove forcibly from power" , "Dethrone or oust" , "Vanquish" , "Defeat completely" .)