End of term decided on by start of last period (3,4)
I believe the answer is:
due date
'end of term' is the definition.
'due date' can be an answer for 'term' (I've seen this before). I'm unsure of the 'end of' bit.
'decided on by start of last period' is the wordplay.
I cannot really understand how this works, but
'on' could be 'at' (I've seen this in other clues) and 'at' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for due date that I've seen is " Term".)