Last of seven days? (3)
I believe the answer is:
end
'last' is the definition.
(I know that last can be written as end)
'of seven days?' is the wordplay.
'of' says the answer is hidden in the clue.
'END' is hidden within 'seven days'.
(Other definitions for end that I've seen before include "Novel by Graham Greene" , "Complete" , "Cessation, conclusion" , "Climax" , "SECOND PART OF 24" .)