Article subsequently cut to form additional item (5)
I believe the answer is:
annex
'additional item' is the definition.
(eg an additional building)
'article subsequently cut' is the wordplay.
I cannot quite understand how this works, but
'article' could be 'an' (indefinite article) and 'an' is found in the answer.
This may be the basis of clue (or it may be nonsense).
'to form' is the link.
Can you help me to learn more?
(Other definitions for annex that I've seen before include "Take possession of (territory)" , "An addition that extends a main building" , "Add by conquest or occupation" , "Attach(ment)" , "Join - add" .)