Hidden word clue
A hidden word clue is a cryptic crossword clue in which the answer is spelled out letter by letter inside the clue's own wording, running straight through the gaps between words.
An indicator marks it: some, part of, in, within, held by, covered by, a bit of, sections of. The answer sits unbroken across word boundaries, which is exactly what makes it hard to see — the eye reads words, not letter runs.
The clue "Some common ionic vegetable (5)" gives ONION, hidden across commON IONic. Some is the indicator, vegetable is the definition, and the letters have not been moved at all.
There is a reversed variant, signalled by an indicator such as going back, brought up or returning in a down clue, where the hidden letters read backwards through the clue. Hidden word clues are usually the first cryptic device a new solver learns to spot, because once you start scanning across the spaces rather than along the words, they leap out.