Heroic tale about, yes, Spanish tennis player (6)
I believe the answer is:
agassi
'spanish tennis player' is the definition.
The definition suggests a singular noun which matches the answer.
'heroic tale about yes' is the wordplay.
'heroic tale' becomes 'saga' (saga is a kind of heroic tale).
'about' shows that the letters should be reversed in order.
'yes' becomes 'si' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'saga' in reverse letter order is 'agas'.
'agas'+'si'='AGASSI'
Can you help me to learn more?
(Other definitions for agassi that I've seen before include "tennis star" , "Andre --, tennis player" , "champion of the court?" , "Andre --, tennis champ" .)