Famous horse corrected evil in small rook (6)
I believe the answer is:
silver
'famous horse' is the definition.
(horse of the fictional character the Lone Ranger)
'corrected evil in small rook' is the wordplay.
'corrected' indicates anagramming the letters.
'in' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'rook' becomes 'r' (chess abbreviation).
'evil' anagrammed gives 'ilve'.
's'+'r'='sr'
'ilve' placed inside 'sr' is 'SILVER'.
(Other definitions for silver that I've seen before include "See 1" , "Element (Ag)" , "Precious stuff" , "Precious metal (for second place)" , "Second-rank medal" .)