CLOSE PROX ACR-LGT GA-SMA EAST OF LAX AT BASE OF TCA.

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AT 16 NM ON THE ILS DME OF LAX; LAX APCH CTL CLRED FLT TO 1900'; HDG 230 DEGS; TO INTERCEPT THE LAX 25L LOC. WHILE ESTABLISHED INBND ON THE RWY 25L LAX LOC AT 2000' AND 12 NM; A RED AND WHITE SMA PASSED APPROX 200' DIRECTLY BENEATH OUR NOSE ON A NORTHERLY HDG. I THINK THE PROB WAS CAUSED BY APCH CTLR CLRING US TO AN ALT THAT WOULD CAUSE US TO EXIT THE FLOOR OF THE TCA (WHILE STILL ON AN IFR CLRNC). I CALLED APCH CTL AFTER WE LANDED. APCH CTLR SUPVR STATED THAT NORMALLY THEY DO NOT ISSUE CLRNCS FOR ACR TO PENETRATE THE TCA FLOOR. HE WOULD CHK WITH THE CTLR TO SEE WHY THIS PARTICULAR CLRNC WAS ISSUED. IF OUR DSCNT WAS MORE RAPID A MIDAIR COLLISION COULD HAVE RESULTED IN A LEGAL SITUATION WHERE THE VFR SMA PLT WAS LEGAL TO FLY BENEATH THE FLOOR OF THE TCA AND; HAD I NOT SCANNED THE HORIZON WHEN I DID; OUR ACR FLT WOULD HAVE BEEN LEGAL AND BUSY COMPLYING WITH THE INSTRUCTIONS RECEIVED. THIS IS A TRAP. TELL APCH CTLS NOT TO VECTOR OR CLR ACFT FOR APCHES WHERE THE ACFT WILL EGRESS AND INGRESS THE TCA ALL ON THE SAME APCH. OR; MAKE IT MANDATORY FOR APCH CTL TO TELL ACFT IF THEY WILL BE EXITING THE TCA FOR A SHORT TIME WHILE BEING VECTORED FOR AN APCH.