A cryptic double definition is, as the name suggests, a type of clue where the solution is defined more than once.

For example:

Get out of the way of American city car (5)

In this case

• American city car = Dodge (remember those?).
• Get out of the way = Dodge.

Like with all other crpytic clues, the double definition may make grammatical sense as a full sentence, but that is always just a red herring.

And beware, double definitions could even be triple definitions, as in:

Procedure for making a hole in cloth (5)

In this case

• Procedure = (A) Drill.
• Making a hole = (To) Drill.
• Cloth = Drill.

Now try this clue:

If you are stumped, give it to the machine!