Greek character has position that's missing opening variation (8)
I believe the answer is:
mutation
'variation' is the definition.
(I know that variation can be written as mutation)
'greek character has position that's missing opening' is the wordplay.
'greek character' becomes 'mu' (letter from the Greek alphabet).
'has position' becomes 'tati' (I am not sure about this - if you are sure you should believe this answer much more).
'that' becomes 'yon' (archaic term meaning 'that').
'missing opening' means to remove the first letter.
'yon' with its initial letter taken off is 'on'.
'mu'+'tati'+'on'='MUTATION'
Can you help me to learn more?
(Other definitions for mutation that I've seen before include "Change in genetic material" , "17 change in form" , "Natural change process" , "Genetic shift" , "Variant form (of gene)" .)