Number bunked off in gutless endeavour (6)
I believe the answer is:
twenty
'number' is the definition.
(twenty is a kind of whole number)
'bunked off in gutless endeavour' is the wordplay.
'bunked off' becomes 'went' (bunk off can mean to leave something).
'in' indicates putting letters inside.
'gutless' suggests removing the centre.
'endeavour' becomes 'try' (synonyms).
'try' with its middle taken out is 'ty'.
'went' put within 'ty' is 'TWENTY'.
(Other definitions for twenty that I've seen before include "Number" , "Hundredweights to a ton" , "Cardinal" , "made by Q on a light blue square" , "with two crosses" .)