Possible Clues:

• Introduction of safety cap for each plug
• Put on a bottle, is it there to stay?
• Plug for a bottle
• Goalkeeper's bung
• ... a ghost that won't let you have any fun?
• One does not continue giving a plug
• Plug for Sunday hat
• It's causing a hold-up in some bottle-neck
• Cork or plug
• Plug organised sport, including fitness training