Worn out as he taxed us horribly (9)
I believe the answer is:
exhausted
'worn out' is the definition.
(exhausting is a kind of wearing out)
'he taxed us horribly' is the wordplay.
'horribly' indicates an anagram.
'he'+'taxed'+'us'='hetaxedus'
'hetaxedus' anagrammed gives 'EXHAUSTED'.
'as' acts as a link.
(Other definitions for exhausted that I've seen before include "Dead-beat" , "All in, really tired" , "spent" , "ready for bed" , "Worn out" .)