I love these little crossword quirks, and 'frail' is one of those clues that can go in several directions depending on the puzzle's mood. For short fill, puzzle-makers most often pick tight, common synonyms like WEAK, PUNY, or SICK — short, unambiguous, and great for intersecting words. If the grid wants a slightly more elevated tone they'll use FEATBLE? No—scratch that—FEABLE isn't a word; they actually go with FEABLE? Wait, the real common one is FEEBLE (6). For objects or glassware you’ll often see FRAGILE (7) or DELICATE (8), while for an elderly person the setter might prefer AGED or INVALID (7) if the clue context leans toward health.
When I'm solving, I always look at part of speech and crossings first. If the clue reads simply 'frail' with no other hint, I default to WEAK or PUNY for 4 letters, FEEBLE for 6, and FRAGILE or DELICATE when the clue suggests fragility of objects. For shorter gimmicks, BONY (4) sometimes sneaks in if the theme hints at skeletal thinness, and RICKETY (7) turns up when the subject is a piece of furniture or structure. Knowing these common lengths helps more than thumbing through a thesaurus — the crosses usually decide between two near-synonyms, and setters tend to favor the more everyday term. It's fun to watch which word wins in different puzzles; personally, I always smile when a clever theme forces an unexpected choice like SICKLY or SLIGHT, because it shows the setter's sly intent.