Invalidate article Greek character left (5)
I believe the answer is:
annul
'invalidate' is the definition.
(I know that invalidate can be written as annul)
'article greek character left' is the wordplay.
'article' becomes 'an' (indefinite article).
'greek character' becomes 'nu' (letter from the Greek alphabet).
'left' becomes 'l' (common abbreviation).
'an'+'nu'+'l'='ANNUL'
(Other definitions for annul that I've seen before include "Scrap" , "Cancel, abolish" , "Invalidate" , "Countermand" , "Cancel, make void" .)