Broadest humour you'll get out of Desmond (6)
I believe the answer is:
widest
'broadest' is the definition.
('wide' can be a synonym of 'broad')
'humour you'll get out of desmond' is the wordplay.
I cannot really see how this works, but
'desmond' could be 'des' (short for Desmond) and 'des' is found within the answer.
'humour' could be 'wit' (synonyms) and 'wit' is found in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for widest that I've seen before include "showing maximal gap between sides" , "most off target" , "most unplayable for batsman?" , "most spacious" , "Most broad" .)