Keep to right entering last passage (7)
I believe the answer is:
stretto
'last passage' is the definition.
(musical passage at the end of an aria)
'keep to right entering' is the wordplay.
'keep' becomes 'stet' (I can't justify this - if you can you should believe this answer much more).
'right' becomes 'r' (common abbreviation).
'entering' means one lot of letters goes inside another.
'stet'+'to'='stetto'
'stetto' going around 'r' is 'STRETTO'.
Can you help me to learn more?
(Other definitions for stretto that I've seen before include "passage quicker" , "Fugal passage" , "Passage in music" , "In quicker time (music)" .)