In a reversal type clue you need to find a word that when reversed (i.e. read backwards) will produce another word.
Here is a typical reversal-type clue:
Extracted backward jeans (5)
These clues generally consist of three parts:
In this case:
Extracted, when read backwards, becomes Denim.
There aren't that many words that read backwards into other words. So you tend to find reversal-type clues combined with other clues into a bigger clue.
Now try this clue:
If you are stumped, give it to the machine!