Look to guarantee a legal document (6,7)
I believe the answer is:
search warrant
'a legal document' is the definition.
(document allowing a search to take place)
'look to guarantee' is the wordplay.
'look' becomes 'search' (synonyms).
'to guarantee' becomes 'warrant' ('warrant' can be a synonym of 'guarantee').
'search'+'warrant'='SEARCH-WARRANT'
(Other definitions for search warrant that I've seen before include "legal document" , "permit to probe" , "Permission to look for stolen goods" , "Police right of entry" .)