Portion of document, it legitimises warrant (7)
I believe the answer is:
entitle
'warrant' is the definition.
(I've seen this before)
'portion of document it legitimises' is the wordplay.
'portion of' indicates the answer is hidden within the clue.
'ENTITLE' is hidden in the letters of 'document it legitimises'.
(Other definitions for entitle that I've seen before include "Give one the right - name" , "Allow" , "Name; empower" , "Name, authorise" , "Warrant" .)