He built right through congested city (4)
I believe the answer is:
wren
'he built' is the definition.
(architect Sir Christopher Wren)
'right through congested city' is the wordplay.
'right' becomes 'r' (abbreviation).
'through' is an insertion indicator (inserted letters go through the word).
'congested city' becomes 'wen' (historical term for a large city).
'r' inserted into 'wen' is 'WREN'.
(Other definitions for wren that I've seen before include "one leaving evidence all around London monument" , "Small bird - great architect" , "English church architect, d. 1723" , "Navy girl" , "held up by wings at small scale?" .)