Regret turning back on footwear vandal (8)
I believe the answer is:
saboteur
'vandal' is the definition.
(I've seen this before)
'regret turning back on footwear' is the wordplay.
'regret' becomes 'rue' ('rue' can be a synonym of 'regret').
'turning back' says the letters should be written backwards.
'on' says to put letters next to each other.
'footwear' becomes 'sabot' (sabot is a kind of footwear**).
'rue' reversed gives 'eur'.
'eur' put after 'sabot' is 'SABOTEUR'.
(Other definitions for saboteur that I've seen before include "Vandal" , "One causes damage for political reasons" , "One who causes deliberate damage to plants or machinery" , "Abuser to become a kind of destroyer" , "Rebel" .)