Faithful former wife before decree (5)
I believe the answer is:
exact
'faithful' is the definition.
(thesaurus)
'former wife before decree' is the wordplay.
'former wife' becomes 'ex'.
'before' says to put letters next to each other.
'decree' becomes 'act' (I've seen this in other clues).
'ex'+'act'='EXACT'
(Other definitions for exact that I've seen before include "Strict" , "accurate in all details" , "totally accurate" , "Force out" , "Demand, extort" .)