Cleverest son facing ruin before exam (8)
I believe the answer is:
smartest
'cleverest' is the definition.
('smart' can be similar in meaning to 'clever')
'son facing ruin before exam' is the wordplay.
'son' becomes 's'.
'facing' is a charade indicator (letters next to each other).
'ruin' becomes 'mar' (I've seen this before).
'before' means one lot of letters go next to another.
'exam' becomes 'test' ('test' can be a synonym of 'exam').
's'+'mar'+'test'='SMARTEST'
(Other definitions for smartest that I've seen before include "Most intelligent or most stylish" , "Most clever" , "Most wise" , "Cleverest; most chic" , "Brightest or best dressed" .)