Stuck with 5 to go.
21a What sounds like a standard piece of artillery? (6) ??N?O? BANGOR ?
27a Shrink taking car round course? Just the opposite (8) ???A?I??
29a One, perhaps, of fundamental importance (8) ???D?N??
Any help appreciated
Sorry - got interrupted by the cat playing on the keyboard. I meant to add for "cannon" sounds like "canon", one meaning of which is a body of laws etc.