Rookie English composer divided sides (7)
I believe the answer is:
learner
'rookie' is the definition.
(thesaurus)
'english composer divided sides' is the wordplay.
I cannot really understand how this works, but
'english' could be 'e' (abbreviation) and 'e' is found within the answer.
'composer' could be 'arne' (Thomas Arne, composer of 'Rule Britannia') and 'arne' is located in the answer.
'sides' could be 'lr' (abbreviations for left and right) and 'lr' is located in the remaining letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for learner that I've seen before include "Driver under instruction" , "10 [STUDENT], possibly 11 [BOOKISH]" , "Trainee (driver)" , "Student (of driving?)" , "Tyro" .)