Napoléon casting aside stirrup? (4)
I believe the answer is:
bone
'stirrup?' is the definition.
(the stirrup is a bone of the ear)
'napoleon casting aside' is the wordplay.
'napoleon' becomes 'Bonaparte' (Napoleon Bonaparte).
'casting' is a deletion indicator (as in casting away).
'aside' becomes 'apart' (synonyms).
'bonaparte' with 'apart' removed is 'BONE'.
(Other definitions for bone that I've seen before include "Part of skeleton" , "Ossified material" , "See 6" , "Possibly hammer" , "Eg, rib" .)