With an subtraction-type clue you are looking for a word that, when having one or more letters removed, becomes another word.
Here is a typical subtraction-type clue:
Crime is the result when clergyman loses his head (5)
These clues generally consist of three parts:
In the above clue Parson is a synonym of clergyman. When you remove its head, i.e. its first letter, then you are left with arson, which is, of course, a crime.
Often, as in this case, you will get a hint as to what you need to remove. "Tail" suggests removing the letter(s) at the end. "Heartless" might suggest removing letters in the middle.
But just as often there are no hints... you just have to figure out what to remove!
Now try this clue:
If you are stumped, give it to the machine!