stolen from wikipedia
<h2>How cryptic clues work</h2>In essence, a cryptic clue describes its answer accurately but only when the clue is read in a very devious way. What the clue appears to say when read normally (the surface reading) is almost never anything to do with the answer and is there as a distraction. The challenge is to find a way of reading the clue that leads to the solution.
In a typical clue, the answer is described in two ways, either of which can come first. One part of the clue is a definition, which must exactly match the part of speech and tense of the answer. The second half (the subsidiary indication) describes the answer in terms of wordplay. (The subsidiary indication can be a second definition in the case of double definition clues.) One of the tasks of the solver is to work out where this boundary occurs and insert a mental pause there when reading the clue cryptically. (Sometimes the two parts are joined with a link word or phrase such as "from" or "could be".)
Because a typical cryptic clue describes its answer in detail and often more than once, the solver can usually have a great deal of confidence in the answer once it has been determined. This is in contrast to non-cryptic crossword clues which often have several possible answers and force the solver to use the crossing letters to distinguish which was intended.
Here is an example (taken from The Guardian crossword of Aug 6 2002, set by "Shed").
15D Very sad unfinished story about rising smoke (8)
is a clue for TRAGICAL. This breaks down as follows.
<ul type="square">[*]15D indicates the location and direction (down) of the solution in the grid[*]"Very sad" is the definition[*]"unfinished story" gives "tal" ("tale" with one letter missing, i.e. "unfinished")[*]"rising smoke" gives "ragic" (a "cigar" is a smoke and this is a down clue so "rising" indicates that "cigar" should be written up the page, i.e. backwards)[*]"about" means that the letters of "tal" should be put either side of "ragic", giving "tragical"[*]"(8)" says that the answer is a single word of eight letters.[/list]
There are many "code words" or "indicators" that have a special meaning in the cryptic crossword context. (In the example above, "about", "unfinished" and "rising" all fall into this category). Learning these, or being able to spot them, is a useful and necessary part of becoming a skilled cryptic crossword solver.