Rookie bothered retainer endlessly (7)
I believe the answer is:
trainee
'rookie' is the definition.
(thesaurus)
'bothered retainer endlessly' is the wordplay.
'bothered' indicates an anagram.
'endlessly' means to remove the last letter.
'retainer' with its last letter taken away is 'retaine'.
'retaine' is an anagram of 'TRAINEE'.
(Other definitions for trainee that I've seen before include "Person under instruction" , "As pupil" , "One being instructed" , "A beginner in Aintree" , "21 [TYRO]" .)