Decree in road construction (6)
I believe the answer is:
ordain
'decree' is the definition.
(I know that ordain is a kind of decree)
'in road construction' is the wordplay.
'construction' is an anagram indicator (answer constructed from certain letters).
'in'+'road'='inroad'
'inroad' anagrammed gives 'ORDAIN'.
(Other definitions for ordain that I've seen before include "Make a decree" , "Appoint - admit to holy orders" , "Consecrate as priest" , "Lay down (law)" , "Establish" .)