15 Lex, legal system,goes inside Pus, matter (yukky matter!)
6 I, currenct, (favourite in crosswords) NS - North and South, teammates in bridge ( a guess) and Taller, bigger. Installer, fitter.
17a is CAP in an anagram (high) of SUN. A (percussion) cap is something you put in a toy gun to make a bang when you fire it. I used to use them when I was a kid playing Cowboys and Indians.
Thanks Quisling.
I remember caps - used to come in a small red rolled-up strip of paper... when i got bored with putting them in a toy gun, I used to hit them with a hammer or ignite them by flicking my fingernail against them, to make them go BANG.