Anagram indicator
An anagram indicator is the word or phrase in a cryptic crossword clue that signals the letters next to it must be rearranged to make the answer.
An indicator has to carry a sense of disturbance for the clue to be fair, so setters draw on words meaning broken, mixed, cooked, moved, damaged, drunk, confused or unusual: broken, cooked, wild, at sea, out, awkwardly, in a mess. The letters being rearranged are called the fodder, and they must sit directly beside the indicator.
Take the clue "Cooked meats produce vapour (5)". The indicator is cooked, the fodder is MEATS, and the definition is produce vapour. Rearranged, MEATS gives STEAM — five letters, as the bracket promised.
Two fairness tests catch most bad clues. The fodder must have exactly as many letters as the answer, so a five-letter answer cannot come from six letters of fodder. And the indicator must plausibly mean disturbance in ordinary English — a setter who uses a word that merely sits next to the fodder without suggesting movement has written an unfair clue, and solvers will say so.