Blunder that hurts knight meeting end (3,4)
I believe the answer is:
own goal
'blunder' is the definition.
'that hurts knight meeting end' is the wordplay.
'that hurts' becomes 'ow!'.
'knight' becomes 'N' (chess abbreviation).
'meeting' says to put letters next to each other.
'end' becomes 'goal' (end can mean one's purpose or goal).
'ow'+'n'+'goal'='OWN GOAL'
(Other definitions for own goal that I've seen before include "Football mishap" , "Defender's faux pas" , "aids the opposition" , "Action that backfires" , "Unintentional self-inflicted harm" .)