Poor grades blocking musical trainees (6)
I believe the answer is:
cadets
'trainees' is the definition.
(cadet is a kind of trainee)
'poor grades blocking musical' is the wordplay.
'poor grades' becomes 'de' (I've seen this in other clues).
'blocking' is an insertion indicator (block can mean to plug a gap in something).
'musical' becomes 'cats' (Andrew Lloyd Webber musical).
'de' put within 'cats' is 'CADETS'.
(Other definitions for cadets that I've seen before include "Army officer trainees" , "Military trainees" , "Casted as young army men" , "Officers potentially" , "Pupils" .)