Starts to write report exonerating northern architect (4)
I believe the answer is:
wren
'architect' is the definition.
(Sir Christopher Wren)
'starts to write report exonerating northern' is the wordplay.
'starts to' says to take the initial letters.
The initial letters of 'write report exonerating northern' is 'WREN'.
(Other definitions for wren that I've seen before include "Small British bird" , "Small active brown northern hemisphere bird" , "Bird - former British servicewoman" , "seafarer" , "See 7 Down" .)