This is a charade - where separate word parts are joined together to form the answer. Look for different sections of the clue that each contribute letters or word segments.
Break this into parts: 'One in five' gives you a word for a member of a group of five, and 'heading to Open are' means take the first letters of those two words. Combine these parts together.
Answer: QUINOA | QUIN (short for quintuplet, meaning 'one in five') + O + A (first letters/headings of 'Open' and 'are') = QUINOA (edible seeds)