Excessive, but not expected yet? (5)
I believe the answer is:
undue
'excessive' is the definition.
('undue' can be a synonym of 'excessive')
'not expected yet?' is the wordplay.
'not' becomes 'un' (I've seen this in other clues).
'expected yet?' becomes 'due' (I've seen this before. I am not sure about the 'yet?' bit.**).
'un'+'due'='UNDUE'
'but' is the link.
(Other definitions for undue that I've seen before include "too much" , "Beyond normal limits" , "Excessive amount" , "More than appropriate" , "Improper" .)