Blunder that hurts: no good, old boy! (3,4)
I believe the answer is:
own goal
'blunder' is the definition.
'that hurts no good old boy' is the wordplay.
'that hurts' becomes 'ow!'.
'no good' becomes 'ng' (abbreviation).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'boy' becomes 'al'.
'ow'+'ng'+'o'+'al'='OWN GOAL'
(Other definitions for own goal that I've seen before include "Football score at the wrong end" , "Effort that backfires" , "that the opposition may thank you for?" , "Blunder penalising the perpetrator" , "Action that backfires" .)