Hand on knight, a chess piece (4)
I believe the answer is:
pawn
'a chess piece' is the definition.
(pawn is a kind of chess piece)
'hand on knight' is the wordplay.
'hand' becomes 'paw' ('paw' can be a synonym of 'hand').
'on' says to put letters next to each other.
'knight' becomes 'n' (chess**).
'paw'+'n'='PAWN'
(Other definitions for pawn that I've seen before include "piece on board" , "Part with temporarily to raise money" , "A dupe; hock" , "Person one used" , "The foot-soldier in chess" .)