On the surface, one knocked out (5)
I believe the answer is:
divot
'out' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'on the surface one knocked' is the wordplay.
'on' becomes 'to'.
'the surface' becomes 'vid' (I can't explain this - if you can you should believe this answer much more).
'one knocked' is a reversal indicator (I've seen 'knocking' mean this).
'to'+'vid'='tovid'
'tovid' back-to-front is 'DIVOT'.
Can you help me to learn more?
(Other definitions for divot that I've seen before include "Golfer's dislodged turf" , "Piece of turf dug out of a lawn" , "Turf lifted by golf club" , "Gouged-out turf" , "Displaced grass" .)