King Crimson with a large following (6)
I believe the answer is:
alfred
'king' is the definition.
(I've seen this before)
'crimson with a large following' is the wordplay.
'crimson' becomes 'red' (synonyms).
'with' means one lot of letters go next to another.
'large' becomes 'L' (eg in clothes sizes).
'following' becomes 'f' (used when citing page numbers etc).
'a'+'l'+'f'='alf'
'red' put after 'alf' is 'ALFRED'.
(Other definitions for alfred that I've seen before include "Former king" , "King after Ethelred" , "Ninth century king of Wessex" , "Saxon king" , "Great English king" .)