Riled bears with disconcerting show of aggression (5-8)
I believe the answer is:
sabre-rattling
'show of aggression' is the definition.
(sabre-rattling means aggressive behaviour)
'riled bears with disconcerting' is the wordplay.
'riled' is an anagram indicator (letters must be irritated or riled up).
'with' means one lot of letters go next to another.
'disconcerting' becomes 'rattling' (rattle can mean to unnerve or disconcert).
'bears' with letters rearranged gives 'sabre'.
'sabre'+'rattling'='SABRE-RATTLING'
(Another definition for sabre-rattling that I've seen is " Aggressive display of military power".)