Aintree, difficult for an apprentice (7)
I believe the answer is:
trainee
'an apprentice' is the definition.
(thesaurus)
'aintree difficult' is the wordplay.
'difficult' indicates an anagram.
'aintree' is an anagram of 'TRAINEE'.
'for' acts as a link.
(Other definitions for trainee that I've seen before include "New recruit" , "One being taught a job" , "Apprentice" , "One learning a job" , "one needs to practice" .)