Defender getting sack is to have adverse consequences (8)
I believe the answer is:
backfire
'to have adverse consequences' is the definition.
(to backfire is to have bad consequences)
'defender getting sack' is the wordplay.
'defender' becomes 'back' (I've seen this before).
'getting' says to put letters next to each other.
'sack' becomes 'fire' (synonyms).
'back'+'fire'='BACKFIRE'
'is' is the link.
(Other definitions for backfire that I've seen before include "Have the opposite and damaging effect to that intended" , "explosion" , "Rebound badly" , "Cause a result opposite to that intended" , "report" .)