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Haven't needed any help for last couple of weeks, but stumped on last six:
15a Scots marked with ruddle vegetable returned to drug user not accepted (6) ?EE?E?
16a South African and Nigerian upset about... (4) ?OE?
17a ...working in disgusting trade of church's income (6) ?IM?N?
4d Sinned, putting bishop in pen (5) ?R?E?
5d Silently reflect about old food in opening of canal (6) ??AT?S
7d Undoubtedly, French investigators waste time (4) ?UR?
Thematic parts contain an extra letter.
Gentlest of hints appreciated

Might 16 be BOER? Def "South African", RE + OB backwards. I've not found an "OB" as a Nigerian, though IBOs are.

17 might be SIMONY?

4d might be ERRED (RR or "Right Reverend" is a bishop)

(This is harder than my usual crosswords, so I'm sorry I can't supply full parsings!)

7d what is the French equivalent of Scotland Yard?

Thanks Chrise! a lot here for me to be getting on with