I name grand levy first that is onerous (6)
I believe the answer is:
taxing
'onerous' is the definition.
('taxing' can be a synonym of 'onerous')
'i name grand levy first' is the wordplay.
'name' becomes 'n' (common abbreviation e.g. on forms).
'grand' becomes 'g' ('G' can be a synonym of 'grand').
'levy' becomes 'tax' (tax is a kind of levy**).
'first' is a charade indicator (letters next to each other).
'i'+'n'+'g'='ing'
'ing' put after 'tax' is 'TAXING'.
'that is' is the link.
(Other definitions for taxing that I've seen before include "hard work" , "Making heavy demands, like Revenue Commissioners" , "Making great demands" , "'Not easily borne, very wearing (6)'" , "Physically or mentally demanding" .)