Some anger at Iona legend's reasoning (9)
I believe the answer is:
rationale
'reasoning' is the definition.
(thesaurus)
'some anger at iona legend's' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'RATIONALE' is hidden in the letters of 'anger at iona legends'.
(Other definitions for rationale that I've seen before include "grounds?" , "Good reason" , "Logical basis (for a course of action)" , "Basis of logic" , "Reasoned exposition" .)