Former decree to be precise (5)
I believe the answer is:
exact
'precise' is the definition.
(I know that precise can be written as exact)
'former decree' is the wordplay.
'former' becomes 'ex' (prefix meaning former or onetime).
'decree' becomes 'act' (I've seen this before).
'ex'+'act'='EXACT'
'to be' is the link.
(Other definitions for exact that I've seen before include "Completely accurate" , "dead" , "Demand, extort" , "Spot on, precise" , "Strict" .)