Precise old law? (5)
I believe the answer is:
exact
'precise' is the definition.
(I know that precise can be written as exact)
'old law?' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'law?' becomes 'act' (law is a kind of act).
'ex'+'act'='EXACT'
(Other definitions for exact that I've seen before include "Strict" , "Precisely correct" , "Absolutely right" , "Precise, accurate" , "Meticulous" .)