8 down: Bent guys with Mr Grouch (6, 4)
Perhaps it’s not the most elegantly created clue, but the answer’s funny! Here, Mr = mister so that bent guys with Mr = misery guts = grouch.
19 across: Lolita’s lover executed cautious tone in shed (7)
This one’s pretty easy because of the direct clue, but the indirect part is nice. Cautious tone = amber, executed (as in behead) cautious tone = mber, shed = hut and executed cautious tone in shed = humbert = Lolita’s lover.
23 down: Lab assistant failing to open American zip (4)
Thankfully, TH came up with Igor as the stereotypical hunchback lab assistant to the crazies, otherwise we would have had great difficulty in deciphering American zip = vigor, failing to open American zip = igor = lab assistant.
21 down: Anteater ridge extended by widowed zookeeper (5)
This one took embarrassingly long to answer, and TH even thought of the solution without saying it because it didn’t seem to make sense. But we both pay enough attention to the television to know that Terri is the name of Steve Irwin’s wife, and that in the middle of anteater ridge you can find terri.