A monocle was an eyeglass used by people with poor vision. It was worn over one eye, often seen as a sign of dignification.
Wally B. Feed was known to wear one and was all but blind without it.
Threepwood can steal it in the moments when Feed takes it off and rubs his eyes.
Threepwood can give the monocle to Captain Dread as a replacement for his missing lucky medallion.