Second article removed from book, declare invalid (5)
I believe the answer is:
annul
'declare invalid' is the definition.
(I've seen this before)
'second article removed from book' is the wordplay.
I cannot quite see how this works, but
'article' could be 'an' (indefinite article) and 'an' is located in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for annul that I've seen before include "Scrap" , "Cancel, abolish" , "Quash" , "Cancel, make void" , "Cancel, revoke" .)