A court ultimately enforces decisions of Parliament (4)
I believe the answer is:
acts
'decisions of parliament' is the definition.
(as in acts of parliament)
'a court ultimately enforces' is the wordplay.
'court' becomes 'ct' (abbreviation used in road names).
'ultimately' says to take the final letters.
The last letter of 'enforces' is 's'.
'a'+'ct'+'s'='ACTS'
(Other definitions for acts that I've seen before include "Religious book" , "Cast in major play sections" , "exerts influence" , "Behaves or plays" , "Legal documents" .)