He caught nothing; left it again with a metric unit of volume (10)

'volume' is the definition.
The answer and definition can be both quantities as well as being singular nouns.
Maybe you can see an association between them that I can't see?

'he caught nothing left it again with a metric unit' is the wordplay.
'caught nothing left' becomes 'col' (I can't justify this - if you can you should believe this answer much more).
'again' becomes 're' (prefix meaning 'again').
'with' is an insertion indicator.
'a metric unit' becomes 't' (t is a kind of metric unit).
'col'+'it'+'re'='colitre'
'colitre' going around 't' is 'ctolitre'.
'he'+'ctolitre'='HECTOLITRE'

'of' is the link.

Can you help me to learn more?