4 interconnected answers:
12A Where top cuts may be bought (7,4) _ A _ _ _ R _ / S _ O _ I can see it's shop. The 4th letter is the initial letter of 13D Bellows B flat, perhaps (6) _ _ O _ _ _ .
17A Driving shift (4,5) _ E _ R / _ _ _ _ _
Gear stick, gear change? giving me a C or a G for 18D Sin moving up (4) _ _ I _