Am about to train extraordinary tennis player (7)
I believe the answer is:
'tennis player' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'am about to train extraordinary' is the wordplay.
'about' shows that the letters should be reversed in order.
'to' is a charade indicator (letters next to each other).
'extraordinary' indicates anagramming the letters.
'am' back-to-front is 'ma'.
'train' is an anagram of 'rtina'.
Can you help me to learn more?
(Other definitions for martina that I've seen before include "Rain mat (anag) - girl's name" , "woman's name" .)