Wiktionary
a sprat to catch a mackerel
n. One may sacrifice something of little value in the hope of gaining something better
Usage examples of "a sprat to catch a mackerel".
A man could not be jealous of a woman whom he regarded as a mere tool, a sprat to catch a mackerel, as the saying had it.
In fact now that he was getting the help he needed - and especially now that he had gone, as it were, on to Garrisons payroll - he had started to think of Terri as something more than merely a sprat to catch a mackerel.
Now that he realized that Vesper had only been a sprat to catch a mackerel the whole picture became clear.