It's sometimes confused with less fine vessel (5)
I believe the answer is:
fewer
'it's sometimes confused with less' is the definition.
I can't tell whether this defines the answer.
'fine vessel' is the wordplay.
'fine' becomes 'f' (abbreviation in pencil classifications).
'vessel' becomes 'ewer' (ewer is a type of jug).
'f'+'ewer'='FEWER'
Can you help me to learn more?
(Other definitions for fewer that I've seen before include "Less in number" , "Not so many" , "in smaller numbers" , "Reduced" , "smaller number of people" .)