New Aintree apprentice (7)
I believe the answer is:
trainee
'apprentice' is the definition.
(I've seen this before)
'new aintree' is the wordplay.
'new' indicates anagramming the letters.
'aintree' anagrammed gives 'TRAINEE'.
(Other definitions for trainee that I've seen before include "Student" , "One being taught a job" , "As pupil" , "One taking instruction" , "One being instructed" .)