Predatory dealer wanting first go (4) H?RK. Maybe SHARK minus the first letter but what about the rest?
Declare old district deserted (5) OREA?. OREAL from O plus Real(M) deserted of one letter.
Poisonous substance alight inside volcanic hill ?ON?NE is I think CONINE. Cone is a type of volcano but IN meaning alight?
That would make the o wrong. This comes from 'tree's outgrowth round mass (4)' ?M?A. OMMA fits - a type of tree = OMA (groth) around M (mass). SO I think the o stands
Hark' seemed fine to me - 'one meaning in Chambers is 'to go' . The 'shark' is the predatory dealer and 'wanting' means the answer loses the first letter.