Correct tax code extremely unusual (5)
I believe the answer is:
exact
'correct' is the definition.
(similar in meaning)
'tax code extremely unusual' is the wordplay.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'unusual' is an anagram indicator.
'code' with its centre taken out is 'ce'.
'tax'+'ce'='taxce'
'taxce' anagrammed gives 'EXACT'.
(Other definitions for exact that I've seen before include "Strict" , "Force out" , "Explicit" , "Completely accurate" , "Call" .)