Greek character on terminus, having no small change (8)
I believe the answer is:
mutation
'change' is the definition.
(I know that mutation is a type of change)
'greek character on terminus having no small' is the wordplay.
'greek character' becomes 'mu' (letter from the Greek alphabet).
'on' is a charade indicator (letters next to each other).
'terminus' becomes 'station' (train terminus).
'having no' suggests deleting specific letters.
'small' becomes 's' (abbreviation - e.g. clothes size).
'station' with 's' removed is 'tation'.
'mu'+'tation'='MUTATION'
(Other definitions for mutation that I've seen before include "17 change in form" , "Variant form (of gene)" , "Change in genetic structure" , "Change, especially biological or genetic" , "Genetic shift" .)