In furnishing, extremes of utilitarianism but bad taste (9)
I believe the answer is:
indecorum
'bad taste' is the definition.
(indecorum can mean bad taste or bad manners)
'in furnishing extremes of utilitarianism' is the wordplay.
'furnishing' becomes 'decor' (I've seen this in another clue).
'extremes of' means to remove the middle letters (only the letters at the extremes of the word).
'utilitarianism' with its middle removed is 'um'.
'in'+'decor'+'um'='INDECORUM'
'but' acts as a link.
(Other definitions for indecorum that I've seen before include "Naughty behaviour" , "Improper behaviour" , "bad taste" , "Bad behaviour" , "Lack of propriety" .)