Stupendous upstanding citizen wins one bronze (7)
I believe the answer is:
titanic
'stupendous' is the definition.
(thesaurus)
'upstanding citizen wins one bronze' is the wordplay.
'upstanding' is a reversal indicator.
'citizen' becomes 'cit' (old-fashioned abbreviation for citizen).
'wins' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'bronze' becomes 'tan' ('tan' can be a synonym of 'bronze').
'cit' in reverse letter order is 'tic'.
'i'+'tan'='itan'
'tic' going around 'itan' is 'TITANIC'.
(Other definitions for titanic that I've seen before include "In act it became an ill-fated ship" , "Gigantic like doomed ocean liner" , "Great ocean liner sunk by iceberg" , "Of exceptional size or power" , "Huge, epic" .)