I’m not absolutely convinced on the point for a number of reasons, but if it is right, why did VN choose “clystère” (=enema) as the metonym for Chekhov’s “secondary activity”?
Bloodletting and enemas were among the very few procedures in premodern medicine, and I have encountered the term "clyster" as a derogatory nickname for physicians before the 19th century. VN talks about "learned doctors crowding around the Malade Imaginaire with their dog-Latin and gigantic belly-pumps" (Nikolai Gogol, p. 2). On second thought, I should tone down my paraphrasing VN's remark and not say he had claimed Chekhov to have dabbled in medicine (I still can't recall where). It would still be fair to read "clystère de Tchékhov"=hobby-horse (an obsolete and slightly pejorative variant of 'hobby'), which makes also sense in the sentence from LATH.
And if I may be pedantic: enema is the procedure, clyster (in English sp.) is the syringe and/or the nozzle it was performed with.