Architect upsetting some planner wilfully (4)
I believe the answer is:
wren
'architect' is the definition.
(Sir Christopher Wren)
'upsetting some planner wilfully' is the wordplay.
'upsetting' says the letters should be written in reverse.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'nerw' is hidden within 'planner wilfully'.
'nerw' reversed gives 'WREN'.
(Other definitions for wren that I've seen before include "Small singer" , "English church architect, d. 1723" , "Bird - architect, 1632-1723" , "Tiny brown songbird with short erect tail" , "Bird - former British servicewoman" .)