A very small volume of liquid running through highway? (9)
I believe the answer is:
streamlet
'a very small volume of liquid running through highway?' is the definition.
I can't judge whether this definition defines the answer.
'a very small volume of liquid running through highway?' is the wordplay.
'very small volume of liquid' becomes 'ml' (millilitre).
'running through' means one lot of letters goes inside another.
'highway?' becomes 'street' (I can't justify this - if you can you should give a lot more credence to this answer).
'a'+'ml'='aml'
'aml' going inside 'street' is 'STREAMLET'.
(Other definitions for streamlet that I've seen before include "hardly a river" , "small brook" , "running water" , "Burn" .)