The soldier, filled with a terrible anger, is ready (8)
I believe the answer is:
sergeant
'the soldier' is the definition.
(I've seen this before)
'filled with a terrible anger is ready' is the wordplay.
'filled with' is an insertion indicator.
'a terrible' indicates anagramming the letters.
'is ready' becomes 'set' (I've seen this before**).
'anger' with letters rearranged gives 'rgean'.
'rgean' placed inside 'set' is 'SERGEANT'.
(Other definitions for sergeant that I've seen before include "Rank above corporal" , "Soldier and lover in Hardy novel" , "NCO; Death a fell one, Hamlet" , "CARRY ON FILM" , "Army rank" .)