All quick crossword compilers secretly want to be cryptic crossword compilers. So you will often find the double definition clue in both types of crossword.
The extra information provided by a second definition gives the solver a helping hand. In a simple category item definition clue such as:
2. Vehicle (3)
the solver has no way to choose between solutions CAR, BUS and VAN until other solved clues in the grid provide a helpful consonant.
The double definition clue solves this problem and is the ancestor of the cryptic clue. The solution is defined in two ways. In a simple crossword these definitions will usually be helpfully separated by a semi-colon or an em-dash:
2. Front (military slang); Vehicle (3)
A cryptic compiler writing a double definition clue for the same answer would leave out the helpful information and punctuation and perhaps throw in a little extra misdirection by making the first word more ambiguous:
2. Lead vehicle (3)
When you see a two word clue in a cryptic crossword, you should mentally fill in the semi-colon. You are not being asked to find a solution for the whole clue. Rather, you are being asked to find the same solution for each half of the clue.