In both cases, the adjective refers to burglars!
- 02.047 A really first-class
- 06.014 If they had still doubted that he was really a first-class burglar,
In OED, both hyphenated and spaced are found in the examples.
“first class, n., adj., and adv.” OED Online, Oxford University Press, June 2017, http://www.oed.com/view/Entry/70612. Accessed 13 September 2017.