Red woollen jacket, new key line (8)
I believe the answer is:
cardinal
'red' is the definition.
(I know this)
'woollen jacket new key line' is the wordplay.
'woollen jacket' becomes 'cardi' (short for cardigan).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'key line' becomes 'al' (I am not sure about this - if you are sure you should believe this answer much more).
'cardi'+'n'+'al'='CARDINAL'
(Other definitions for cardinal that I've seen before include "first" , "Prince of the Roman Catholic Church" , "Main - church dignitary" , "One, perhaps" , "Leading RC dignitary" .)