A knight trailing behind king, hard ruler (4)
I believe the answer is:
khan
'ruler' is the definition.
(historic title in certain Muslim countries)
'a knight trailing behind king hard' is the wordplay.
'knight' becomes 'N' (chess abbreviation).
'trailing' is a charade indicator (letters next to each other).
'behind' means one lot of letters go next to another.
'king' becomes 'K' (abbreviation for king in chess).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'a'+'n'='an'
'k' next to 'h' is 'kh'.
'an' put after 'kh' is 'KHAN'.
(Other definitions for khan that I've seen before include "Caravanserai; Asian ruler" , "Title of central Asian rulers" , "Central Asian ruler" , "Imran --, cricketer" , "Eastern potentate" .)