Taxing individual with sour disposition (7)
I believe the answer is:
onerous
'taxing' is the definition.
('onerous' can be a synonym of 'taxing')
'individual with sour disposition' is the wordplay.
'individual' becomes 'one' (**).
'with' means one lot of letters go next to another.
'disposition' indicates an anagram.
'sour' is an anagram of 'rous'.
'one'+'rous'='ONEROUS'
(Other definitions for onerous that I've seen before include "Heavy, difficult to do or bear" , "Involving much effort" , "Burdensome, heavy kind of duty" , "Involving a great deal of effort" , "weight" .)