Not sure about parsing these:
15a. It suggests cleric following it only gets worse (7,6)
A: Vicious circle.
Can't see how the first part fits.
19a. Brief address of surgeon is going around in May (5, 8)
A: Prime minister.
Prime = brief I suppose, but what's the "address of surgeon" doing?
4. Staff in NY opera house love souvenir (7)
A: Momento.
Staff = men, inside MO for the NY opera house and the final "o" for love. Where does the "t" come from?