One politician within the law -- that's understood (8)
I believe the answer is:
implicit
'understood' is the definition.
(I've seen this before)
'one politician within the law' is the wordplay.
'one politician' becomes 'imp' (I've seen this in other clues).
'within the law' becomes 'licit' (I've seen this before).
'imp'+'licit'='IMPLICIT'
'that's' acts as a link.
(Other definitions for implicit that I've seen before include "Implicit" , "present, but not visible" , "Inherent but not made clear" , "Suggested but not stated" , "Unquestioning - expressed indirectly" .)