Precise old law? (5)
I believe the answer is:
exact
'precise' is the definition.
(synonyms)
'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" .)