Successful students will retain edge in percussion instrument (7)
I believe the answer is:
marimba
'percussion instrument' is the definition.
(marimba is a kind of percussion instrument)
'successful students will retain edge' is the wordplay.
I cannot quite see how this works, but
'edge' could be 'rim' (rim is a kind of edge) and 'rim' is found in the answer.
This may be the basis of clue (or it may be nonsense).
'in' is the link.
Can you help me to learn more?
(Other definitions for marimba that I've seen before include "Form of xylophone" , "Deep toned percussion instrument" , "Latin American instrument like a xylophone" , "Jazz instrument" , "African xylophone" .)