A number went into extremes of torpidity (6)
I believe the answer is:
twenty
'a number' is the definition.
(twenty is a kind of number)
'went into extremes of torpidity' is the wordplay.
'into' indicates putting letters inside.
'extremes of' means to remove the middle letters.
'torpidity' with its middle taken out is 'ty'.
'went' inserted inside 'ty' is 'TWENTY'.
(Other definitions for twenty that I've seen before include "Cardinal" , "John B Keane wrote about many young men of this age" , "Dozen plus eight" , "made by Q on a light blue square" , "One score" .)