22a: abbrev. for 'baron' + word for concocts (C is extra)
29a: a famously besieged city + word for 'further' with last letter omitted. (T is extra)
20d: a former university + abbrev. for extreme pressure (E is extra)
24d: a gig is a type of boat.
I've got what looks like a complete satisfactory grid, except that I can only find 4 clashes: One in 1A, two in 6A, and one in 22A.
Where's the 5th?