Knight on silver horse (3)
I believe the answer is:
nag
'horse' is the definition.
(I've seen this before)
'knight on silver' is the wordplay.
'knight' becomes 'n' (chess).
'on' is a charade indicator (letters next to each other).
'silver' becomes 'ag' (Ag is the chemical symbol for silver).
'n'+'ag'='NAG'
(Other definitions for nag that I've seen before include "plague" , "shrew or > badger" , "Keep criticising, hurting" , "Worry; old horse" , "Worry constantly" .)