Nice poet involved with tax? That’s the assumption (11)
I believe the answer is:
expectation
'that's the assumption' is the definition.
(assumption can mean an expectation)
'nice poet involved with tax?' is the wordplay.
'involved with' indicates anagramming the letters (involved can mean confusing or complex).
'nice'+'poet'='nicepoet'
'nicepoet' with its letters anagrammed with 'tax' gives 'EXPECTATION'.
(Other definitions for expectation that I've seen before include "Prospect of future good" , "Prospect (Pip's were great!)" , "Assumption - hope" .)