Possible Clues:

• Unimportant person having to shed pounds to qualify for bout?
• A little extra, Spooner says to enliven partner?
• Filler
• Compel to stay on phone: it supplies a deficiency
• What boxer must do before fight as an unimportant person
• Mum joins team of west London rowers -- one to fill up the number?
• Small addition to make up mass
• Small thing added to make up batch
• Token presence of master rowers circumventing London district
• It helps the balance, though not the figure