STUDENT AND INSTRUCTOR DEPART CMA FOR A NIGHT IFR FLT TO LAS AND EXPERIENCE A DELAY RECEIVING AN IFR CLRNC WHILE ENCOUNTERING DETERIORATING WX CONDITIONS.

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I DEP CAMARILLO ON AN IFR FLT PLAN TO LAS VEGAS. CTR WAS NOT RESPONDING TO MY REQUEST FOR THE IFR CLRNC; SO WE DEPARTED UNDER VFR CONDITIONS; AND PLANNED TO PICK UP THE CLRNC ONCE AIRBORNE. THE WX BRIEFING STATED THAT GOOD VFR CONDITIONS PREVAILED; SO A VFR DEP WAS ACCEPTABLE. HOWEVER; ONCE AIRBORNE CONDITIONS APPEARED TO HAVE DETERIORATED. WHILE MAINTAINING VFR; WE ATTEMPTED TO OBTAIN THE IFR CLRNC; BUT CTR DELAYED IT. RADAR CONTACT WAS NOT ESTABLISHED INITIALLY; AND I STATED WE WOULD BE UNABLE TO MAINTAIN VFR IN THE CLB. AT THIS POINT; CTR QUESTIONED WHETHER WE HAD RECEIVED A WX BRIEFING. BY THE TIME WE DID RECEIVE THE CLRNC; WE WERE IN AN AREA OF VFR; AND REQUESTED A VFR CLB ON COURSE. APPARENTLY; A MARINE LAYER MOVED IN ALONG THE COAST WHICH WAS UNFORECAST AND NOT EASILY DETECTABLE AT NIGHT. FURTHERMORE; HAD WE NOT PROCEEDED TO AN AREA OF GOOD WX; THE INITIAL FLT PLAN ROUTING WOULD HAVE LED US INTO AN AREA OF MARGINAL VFR AT NIGHT IN MOUNTAINOUS TERRAIN. IN SUMMARY; AN IFR CLRNC SHOULD BE OBTAINABLE PRIOR TO DEP; OR EXPEDITIOUSLY UNDER SIMILAR CONDITIONS IN FLT.