Polish individual on horse (4)
I believe the answer is:
hone
'polish' is the definition.
(I've seen this before)
'individual on horse' is the wordplay.
'individual' becomes 'one' (an individual is one person).
'on' means one lot of letters go next to another.
'horse' becomes 'H' (synonyms).
'one' after 'h' is 'HONE'.
(Other definitions for hone that I've seen before include "Sharpen (a blade)" , "Sharpen tool on stone" , "Strop" , "Smooth stone" , "Polish" .)