I should think you can get away with a navy, black or white polo shirt, cream jods and a clean navy or black riding jacket! As long as everything is clean and fairly smart, clear round jumping is usually fairly laid back!
I usually wear smart navy jods & a smart blouson type jacket for clear round, lots of people do wear show gear though so whatever you feel happy in & sometimes its nice to dress up!
I went there for a clear round day earlier in the year and most were in casual wear. A few were in comp gear, but there was also a dressage comp on at the same time, so were popping round the course after their test.
I just wore my smart boots, dark breeches and a smart polo shirt.