The day one took the exam (3)
I believe the answer is:
sat
'exam' is the definition.
(standard attainment test, UK exam)
'the day one took the' is the wordplay.
I cannot really see how this works, but
'one' could be 'a' and 'a' is found in the answer.
The remaining letters 'st' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for sat that I've seen before include "twenty-four hours" , "Took (examination)" , "Was couch potato" , "Was in session" , "nearly half a day" .)