Drinks before start of exam -- a mistake (5)
I believe the answer is:
lapse
'a mistake' is the definition.
(lapse is a kind of mistake)
'drinks before start of exam' is the wordplay.
'drinks' becomes 'laps' (lapping is a kind of drinking).
'before' is a charade indicator (letters next to each other).
'start of exam' becomes 'e' (I can't justify this - if you can you should believe this answer much more).
'laps'+'e'='LAPSE'
Can you help me to learn more?
(Other definitions for lapse that I've seen before include "moral shortcoming" , "Retrogress" , "Cease to be valid" , "Error; oversight" , "A slight error or a passage of time" .)