It is not possible to use a measure to create a new measure. However, what you can do is use variables. https://help.qlik.com/en-US/sense/3.1/Subsystems/Hub/Content/Variables/use-variables-expression.htm
So for example, you could create a variable that you could call 'vSales' which would be defined as Sum(Sales) and a variable called 'vCost' which could be defined as Sum([Costs of Goods])
Then you can create a new variable that you could call vMargin and define it as
($(vSales) - $(vCost)) / $(vSales)
Note the "$" is used to tell Qlik to evaluate the variable in the brackets and return the result.
Create a new measure by clicking "Create new" in your master items.
The expression for your measure is just the following:
Give the measure a name, and optionally a description or tags, and click Create.
That's all there is to it.
There are some technicalities if you are looking to represent varying aggregation levels, or if you are using variables for Labels or in If statements - but most of the time it's pretty simple.