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

'sauce made' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

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

'from' is the link.

Can you help me to learn more?