Make certain last date becomes satisfactory (6)
I believe the answer is:
ensure
'make certain' is the definition.
(synonyms)
'last date becomes satisfactory' is the wordplay.
I cannot really see how this works, but
'satisfactory' could be 's' (abbreviation in education) and 's' is found in the answer.
The remaining letters 'enure' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for ensure that I've seen before include "warrant" , "Make certain, clinch" , "Make certain or safe" , "Make certain for the future" , "Make certain, confirm" .)