Stumble over second Cambridge honours exam (6)
I believe the answer is:
tripos
'cambridge honours exam' is the definition.
(I've seen this before)
'stumble over second' is the wordplay.
'stumble' becomes 'trip' ('trip' can be a synonym of 'stumble').
'over' becomes 'o' (cricket abbreviation).
'second' becomes 's' (common abbreviation**).
'trip'+'o'+'s'='TRIPOS'
(Other definitions for tripos that I've seen before include "Cambridge honours examination" , "Final honours exam for Cambridge BA" , "important examination" , "university exam" .)