Knowing when student fails to finish (6)
I believe the answer is:
astute
'knowing' is the definition.
(thesaurus)
'when student fails to finish' is the wordplay.
'when' becomes 'as'.
'student' becomes 'tutee' (I've seen this before).
'fails to finish' means to remove the last letter.
'tutee' with its final letter taken off is 'tute'.
'as'+'tute'='ASTUTE'
(Other definitions for astute that I've seen before include "Shrewd and sagacious" , "Able to make accurate assessments" , "'Perceptive, shrewd (6)'" , "Shrewd, canny" , "Wise" .)