Unorthodox cast insisted on earlier date (9)
I believe the answer is:
dissident
'unorthodox' is the definition.
(similar in meaning)
'cast insisted on earlier date' is the wordplay.
'cast' is an anagram indicator (letters are dispersed or cast around).
'on' means one lot of letters goes inside another (as in clothing 'on' a person).
'earlier' suggests taking the first letters (I've seen 'earliest of' mean this).
The initial letter of 'date' is 'd'.
'insisted' is an anagram of 'dissient'.
'dissient' enclosing 'd' is 'DISSIDENT'.
(Other definitions for dissident that I've seen before include "Protester" , "Opposer of official line" , "One opposed to official policy" , "Opposer of officialdom" , "One opposing official policy" .)