Presumed some bounder's too dastardly (10)
I believe the answer is:
understood
'presumed' is the definition.
(thesaurus)
'some bounder's too dastardly' is the wordplay.
'some' indicates a hidden word (some parts of the word are used).
'UNDERSTOOD' can be found hidden inside 'bounders too dastardly'.
(Other definitions for understood that I've seen before include "Accepted, implied" , "Taken as 23 [READ]" , "Comprehended" , "Got" .)