Water, but there's a certain amount of wine on the way (5)
I believe the answer is:
seine
'water' is the definition.
(Seine is an example)
'amount of wine on the way' is the wordplay.
'amount' becomes 'I' (I is an example).
'of wine' becomes 'sen' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' means one lot of letters goes inside another (as in clothing 'on' a person).
'the way' becomes 'e' (Compass direction - East).
'i' inserted inside 'sen' is 'sein'.
'sein'+'e'='SEINE'
'but there's a certain' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for seine that I've seen before include "Paris river" , "Paris's river" , "French river -- type of fishing net" , "Parisian river" , "Second longest river in France" .)