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Four I cannot do/am unsure of:

28dn, 'Opening of a test's beginning at the southern end', (5), S?O?A;

51dn, 'Enzyme offers control, containing nitrogen', (5), R?N??;

56ac, 'Not set to race where famous marathon takes place' (5), R???Y; RALLY?

42dn, 'Extreme practice of sexual libertarianism fuelled by drug (9), S???G?I?G;

48ac, 'Skin defect's beginning to disappear, surgeon's found' (6), ???T?R; CUTTER?

51renin?

28 Stoma, opening. MOT is the test, reversed+ S/southern +a, in clue.

56 Runny, not set. Run (in)NY/New York (where they have a marathon).
48 Lister, surgeon (Blister loses its first letter/beginning.)

42 Swingeing, extreme. Swinging (sexual libertarianism) +E/drug.

42. Swingeing

Thank you both.

Glad I could help.