Subversive regrets boxing to a degree, when knocked over (8)
I believe the answer is:
saboteur
'subversive' is the definition.
(thesaurus)
'regrets boxing to a degree when knocked over' is the wordplay.
'regrets' becomes 'rues' ('rue' can be a synonym of 'regret').
'boxing' is an insertion indicator ('box' can mean contain, as in to put in a box).
'a degree when' becomes 'ba' (I can't justify this - if you can you should give a lot more credence to this answer).
'knocked over' says the letters should be written backwards.
'to'+'ba'='toba'
'rues' enclosing 'toba' is 'ruetobas'.
'ruetobas' reversed gives 'SABOTEUR'.
Can you help me to learn more?
(Other definitions for saboteur that I've seen before include "One who wrecks" , "One who disrupts or damages by design" , "He destroys in order to subvert" , "One causing deliberate destruction" , "One causes damage for political reasons" .)