Thanks ixion,
The clues for 6,7 and 8 down are now
6) Combine to figure it out once four dismissed? (3,8) P - T, - O - E - - - R
7)Dogged investigator? (6-5) S - E - T -/H - - - D
8)Not entirely where studying at Oxford, say, may lead (2,1,6) T -, A, - E - R - -