Sort tunes out outside - most unpleasant (9)
I believe the answer is:
unkindest
'most unpleasant' is the definition.
Both the answer and definition are superlative adjectives. Perhaps there's an association between them I don't understand?
'sort tunes out outside' is the wordplay.
'sort' becomes 'kind' (synonyms).
'out' indicates an anagram (out can mean wrong or inaccurate).
'outside' means one lot of letters goes inside another (some letters go outside others).
'tunes' anagrammed gives 'unest'.
'kind' put inside 'unest' is 'UNKINDEST'.
Can you help me to learn more?
(Other definitions for unkindest that I've seen before include "Most brutal" , "Most uncharitable" , "Extremely cruel" .)