Ok guys, where have I gone wrong this week!!
12a A preface to n argument intnded to anticipate and preclude objections(9) ??O?E?S?S Is PROCESSES?
17a Old term meaning quick, eager or early(5) ??T?U
6d A genus of succulent plants of the Crassulaceae family(9) E?H?V???A
15d A pitch-black solid bitumen(9) ??B?R?E?E
I notice that RHOMB is given as the answer to 20a but I can't get that to fit with my 19d for which i've got ...wait for it....CUNTLINE!!!!!