English maiden keeping animal for riding: Black Beauty is one (6)
I believe the answer is:
eponym
'black beauty is one' is the definition.
(the novel Black Beauty is named for the character)
'english maiden keeping animal for riding' is the wordplay.
'english' becomes 'e' (abbreviation).
'maiden' becomes 'm' (cricket abbreviation).
'keeping' means one lot of letters goes inside another.
'animal for riding' becomes 'pony'.
'e'+'m'='em'
'em' enclosing 'pony' is 'EPONYM'.
(Other definitions for eponym that I've seen before include "handle from place?" , "Hero" , "eg 'Jane Eyre'" , "a mackintosh, for instance" , "someone giving a name" .)