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18 Down: Speak vaingloriously (5) B-A--
Cryptic clue: Brag about being leading seaman in a yacht
Last letter gives first letter for 24 Across: Sesame (3) --L
Cryptic clue: Sesame is ignited in reverse
I thought this might be til (lit reversed) but can't find anything to prove it

18 Boast

Boat around S (start of seaman)

24 Yes - Til

http://www.collinsdictionary.com/dictionary/english/til

Thanks very much mamya

You will find TIL referred to here
http://en.wikipedia.org/wiki/Sesame

You're welcome.