Reported soldier's bitterness (7)
I believe the answer is:
rancour
'bitterness' is the definition.
(synonyms)
'reported soldier's' is the wordplay.
'reported' shows a homophone (sound like).
'soldier' becomes 'ranker' (ranker is a kind of soldier).
'ranker' is a homophone of 'RANCOUR'.
(Other definitions for rancour that I've seen before include "generating ill-feeling ..." , "Bitter feeling or ill will" , "Bitter feeling or ill-will" , "Long-lasting bitterness" , "Entrenched enmity" .)