To rush into extremes of turpitude is nonsense! (5)
I believe the answer is:
tripe
'is nonsense' is the definition.
(informal term for nonsense or false information)
'to rush into extremes of turpitude' is the wordplay.
'to rush' becomes 'rip' (I've seen this in other clues).
'into' is an insertion indicator.
'extremes of' means to remove the middle letters (only the letters at the extremes of the word).
'turpitude' with its centre removed is 'te'.
'rip' placed into 'te' is 'TRIPE'.
(Other definitions for tripe that I've seen before include "Ruminant's stomach lining" , "Rhubarb" , "which a freegan might eat?" , "Edible lining of cow's stomach - rubbish" , "Rubbish; stomach as food" .)