Block Liberal legislation in U-turn (4)
I believe the answer is:
wall
'block' is the definition.
(thesaurus)
'liberal legislation in u-turn' is the wordplay.
'liberal' becomes 'l' (abbreviation).
'legislation' becomes 'law' (I've seen this in other clues).
'in' is an insertion indicator.
'u-turn' shows that the letters should be reversed in order.
'l' inserted into 'law' is 'llaw'.
'llaw' backwards is 'WALL'.
(Other definitions for wall that I've seen before include "See 14" , "Boundary of brick or stone" , "mechanical part" , "Brick or stone barrier" , "A challenge for the mountaineer" .)