Instigator of luminarism, as well as visionary artist (8)
I believe the answer is:
landseer
'artist' is the definition.
(Edwin Landseer RA)
'instigator of luminarism as well as visionary' is the wordplay.
'instigator of' says to take the initial letters (the letter which begins or 'instigates' the word).
'as well as' becomes 'and' (I've seen this in another clue).
'visionary' becomes 'seer' (synonyms).
The initial letter of 'luminarism' is 'l'.
'l'+'and'+'seer'='LANDSEER'
(Other definitions for landseer that I've seen before include "See 1 down" , "Artist" , "English (animal) painter, d. 1873" , "Sir Edwin -, Monarch of the Glen painter" , "The Monarch of the_Glen painter" .)