Vassal that is restricted by broken leg (5)
I believe the answer is:
liege
'vassal' is the definition.
('liege' can be a synonym of 'vassal')
'that is restricted by broken leg' is the wordplay.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'restricted by' indicates putting letters inside (I've seen this in other clues).
'broken' indicates anagramming the letters.
'leg' anagrammed gives 'lge'.
'ie' inserted inside 'lge' is 'LIEGE'.
(Other definitions for liege that I've seen before include "European university" , "Large Belgian city" , "Sort of lord" , "Feudal vassal from a Belgian city" , "Belgian, French-speaking city" .)