Excess quantity left in narrow passage (4)
I believe the answer is:
glut
'excess' is the definition.
(glut can mean an excess of something)
'quantity left in narrow passage' is the wordplay.
'quantity' becomes 'G' (G is an example).
'left' becomes 'l' (common abbreviation).
'in narrow passage' becomes 'ut' (I can't justify this - if you can you should give a lot more credence to this answer).
'g'+'l'+'ut'='GLUT'
Can you help me to learn more?
(Other definitions for glut that I've seen before include "Overabundance" , "Excessive amount" , "A surfeit as of crops" , "A surplus or excess" , "stuff" .)