Clue: Highest degree, English, old examination accepted, above board (2,3,5)
Answer: ON THE LEVEL
Explanation: Definition: above board
An envelope ('accepted') of NTH ('highest degree' – as in "to the nth") plus E ('English') in O LEVEL ('old examination'). It is perhaps ambiguous, but I would have thought that 'accepted' here would more likely suggest an envelope in the other way.