Fail to have the desired effect giving defender sack (8)
I believe the answer is:
backfire
'fail to have the desired effect' is the definition.
I can't judge whether this definition defines the answer.
'giving defender sack' is the wordplay.
'giving' is a charade indicator (letters next to each other).
'defender' becomes 'back' (left-backs and right-backs are football defenders).
'sack' becomes 'fire' (both can mean to dismiss from a job).
'back' put next to 'fire' is 'BACKFIRE'.
Can you help me to learn more?
(Other definitions for backfire that I've seen before include "Rebound adversely (on the originator)" , "Have the opposite and damaging effect to that intended" , "Come home to roost" , "Rebound on oneself" , "report" .)