Possible Clues:

• S-word, often
• More than one pedalo at first left on Eurasian river
• Mice or men
• More than one friend accepts bait without finishing
• More than one rook up, all went wrong
• More than one in number, like serous membranes, we're told
• See 21 Down
• Word form made with the big letter depicted in this puzzle's diagram (this letter is entirely absent from the solution)
• Word ending in 'L' for words ending in 'S'?
• Deer or elk, sometimes