Printer's devilry (PD) is a popular cryptic crossword variant in which the clues do not contain a definition or readily identifiable wordplay, but suppose instead that a certain sentence (the undevilled sentence) has had a certain word or phrase removed by the eponymous “printer's devil”; after the letters have been shifted about and punctuation has been possibly added or removed, the resulting sentence is offered up as the clue itself (the devilled sentence). The solver must then ascertain exactly what the printer's devil removed, such that replacing it in the sentence and shifting the letters and punctuation about once more results in the original meaningful and grammatical sentence: this removed word or phrase is then the clue's solution.
It is considered good cluing practice on the part of the setter to leave the solver hints, either by giving part of the devilled sentence slightly odd grammar to suggest the area from which the solution has been taken, or by adding extra words to the devilled sentence to provide additional context for the solution when found.
As a rule, standard (daily) cryptic crossword puzzles do not include printer's devilry-style clues, but “higher tier” crosswords such as The Listener may include them without warning (although the norm is to provide a brief explanation of their mechanics and to forewarn of their presence).
Consider the clue
A galling remark – an effective riposte is difficult (7)
taken from Azed 1888. Here, the solver is expected to find the answer INSTATE, which, when placed back into the (slightly reorganised) original sentence in the boldface position, yields the undevilled version
Against a telling remark, an effective riposte is difficult
a natural, grammatical sentence whose context confirms the answer is correct.