Sycophant is present as duke descends (5)
I believe the answer is:
toady
'sycophant' is the definition.
(I know that sycophant can be written as toady)
'present as duke descends' is the wordplay.
I cannot quite understand how this works, but
'present' could be 'ad' (AD, Anno Domini, as opposed to BC, Before Christ) and 'ad' is located in the answer.
The remaining letters 'toy' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'is' is the link.
(Other definitions for toady that I've seen before include "Curry favour" , "crawler" , "Yes-man" , "fawn" , "Flatterer or sycophant" .)