Heads rolled in final contest, off the mark (6)
I believe the answer is:
astray
'off the mark' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for astray that I've seen before include "Off the correct track" , "Wandering off the track" , "(Gone) in wrong direction" , "Well off course" , "wrong way" .)