Nous about task (4)
I believe the answer is:
onus
'task' is the definition.
(thesaurus)
'nous about' is the wordplay.
'about' is an anagram indicator.
'nous' anagrammed gives 'ONUS'.
(Other definitions for onus that I've seen before include "'Burden, load (4)'" , "Responsibilty" , "Capulet opponent (R&J)" , "Encumbrance" , "Liability" .)