[Just a few to clear up. I think this Xword's too hard for me actually!]
16d]Bum cigars from rear in Anus, successes.[5][NB-Anus is setter]
S?I?S
9/11]Celebration beginning for geriatric poking arse madly-that's mild for 26 across, perhaps.[4,5]
?A??/L?I??
[NB-16d--Anus is setter]
[NB-9/11-26 across answer is PENSIONERS].