Start to swot a month before exam — that's most clever (8)
I believe the answer is:
smartest
'most clever' is the definition.
('smart' can be similar in meaning to 'clever')
'start to swot a month before exam' is the wordplay.
'start to' indicates taking the first letters.
'a month' becomes 'mar' (Mar is an example**).
'before' is a charade indicator (letters next to each other).
'exam' becomes 'test' ('test' can be a synonym of 'exam').
The initial letter of 'swot' is 's'.
's'+'mar'+'test'='SMARTEST'
'that's' is the link.
(Other definitions for smartest that I've seen before include "Extremely bright" , "Brightest or best dressed" , "Most intelligent or most stylish" , "Most wise" , "Most clever" .)