Quantity of fluid, possibly paint (5)
I believe the answer is:
pinta
'quantity of fluid' is the definition.
The definition suggests a singular noun which matches the answer.
'possibly paint' is the wordplay.
'possibly' indicates anagramming the letters.
'paint' with letters rearranged gives 'PINTA'.
Can you help me to learn more?
(Other definitions for pinta that I've seen before include "Bottle of milk" , "drink" , "vessel for Columbus?" , "early delivery?" , "Columbus's ship - some milk!" .)