Influential artist embodied by Spall occasionally (7)
I believe the answer is:
seminal
'influential' is the definition.
(seminal can mean important or influential)
'artist embodied by spall occasionally' is the wordplay.
I cannot really see how this works, but
'artist' could be 'emin' (I've seen this before) and 'emin' is found in the answer.
The remaining letters 'sal' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for seminal that I've seen before include "Menials giving rise to new beginnings" , "Original and influential" , "Strongly influencing future developments" , "Menials (anag) - creative" , "Major" .)