Tautest order in the law (7)
I believe the answer is:
statute
'the law' is the definition.
(I've seen this before)
'tautest order' is the wordplay.
'order' indicates an anagram.
'tautest' is an anagram of 'STATUTE'.
'in' is the link.
(Other definitions for statute that I've seen before include "Law recorded in book" , "Written law" , "Act of law" , "Set taut (anag.)" , "Act of Parliament" .)