All a little tiresome, very onerous (8)
I believe the answer is:
everyone
'all' is the definition.
(I've seen this before)
'a little tiresome very onerous' is the wordplay.
'a little' indicates the answer is hidden within the clue (a little bit of the clue).
'EVERYONE' is hidden in the letters of 'tiresome very onerous'.
(Other definitions for everyone that I've seen before include "Uncle Tom Cobleigh and all" , "All without exception" , "Entire group" , "the masses" , "All the people" .)