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"This caring lark" is Dr Watson. "Not much cop" means "not too much affection". The missing phrase is "given by". So, the full sentence would be: "Not much cop given by this caring lark" and it means that the subject person is not showing affection to the object person although it is generally a caring perosanility. tl;dr Holmes points out how (usually kind) Watson got that pissed off by Holmes that he got cold towards him.