Full-length intro to 'Halloween' contains nothing rude (7)
I believe the answer is:
uncouth
'rude' is the definition.
('uncouth' can be a synonym of 'rude')
'full-length intro to halloween contains nothing' is the wordplay.
'full-length' becomes 'uncut' ('uncut' can be a synonym of 'full-length').
'intro to' suggests taking the first letters (I've seen 'intros to' mean this).
'contains' is an insertion indicator.
'nothing' becomes 'o' (looks like zero - 0).
The initial letter of 'halloween' is 'h'.
'uncut'+'h'='uncuth'
'uncuth' enclosing 'o' is 'UNCOUTH'.
(Other definitions for uncouth that I've seen before include "Boorish - without polish" , "lacking in charm" , "Crude" , "Ill-mannered" , "Lacking good manners" .)