Possible Clues:

• One who won't tell
• It's 1.50 in the morning, so shut up!
• Secretive person - mollusc
• Reticent shellfish?
• Shelley might have shut up at 1.50 in the morning
• This ought to shut up our racket
• Burrowing marine mollusc
• It's one-fifty in the morning, so shut up! (4)
• Seafood morsel
• One disinclined to talk