Sauce made from fish and ... um ... that's it (5)

'sauce made' is the definition.
I know nothing about this answer so I can't judge whether this works.

'fish and um that's it' is the wordplay.
I cannot quite see how this works, but
'fish' could be 'gar' (type of fish) and 'gar' is found within the answer.
'um' is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).

'from' acts as a link.

Can you help me to learn more?