Runner is going back to protect king (3)
I believe the answer is:
ski
'runner' is the definition.
(something which 'runs', ie moves quickly on snow)
'is going back to protect king' is the wordplay.
'going back' says the letters should be written backwards.
'to protect' indicates putting letters inside.
'king' becomes 'k' (abbreviation for king in chess).
'is' reversed gives 'si'.
'si' enclosing 'k' is 'SKI'.
(Other definitions for ski that I've seen before include "Glide ove snow" , "Use runners crossing snow" , "Slide downhill, sportingly" , "Travel across snow" , "Narrow runner for snow and water" .)