Leave the straight and narrow left in the rump (9)
I believe the answer is:
backslide
'leave the straight and' is the definition.
I can't tell whether this defines the answer.
'narrow left in the rump' is the wordplay.
'narrow left' becomes 'l' (common abbreviation. I am not sure about the 'narrow' bit.).
'in the' means one lot of letters goes inside another.
'rump' becomes 'backside' (synonyms).
'l' put into 'backside' is 'BACKSLIDE'.
Can you help me to learn more?
(Other definitions for backslide that I've seen before include "Relapse into bad ways" , "Degenerate" , "Lapse into bad habits" .)