Adding rear sack could make things worse (8)
I believe the answer is:
backfire
'make things worse' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'adding rear sack' is the wordplay.
'adding rear' becomes 'back' (I can't explain this - if you can you should believe this answer much more).
'sack' becomes 'fire' (both can mean to dismiss from a job).
'back'+'fire'='BACKFIRE'
'could' acts as a link.
Can you help me to learn more?
(Other definitions for backfire that I've seen before include "Come home to roost" , "Rebound adversely (on the originator)" , "Have the opposite and damaging effect to that intended" , "Rebound on oneself" , "Go wrong" .)