Precision of a Christian creed laid down by saints (8)
I believe the answer is:
niceness
'precision' is the definition.
(thesaurus)
'a christian creed laid down by saints' is the wordplay.
'a christian creed' becomes 'nicene' (I've seen this in another clue).
'laid down by' says to put letters next to each other.
'saints' becomes 'ss'.
'nicene'+'ss'='NICENESS'
'of' is the link.
(Other definitions for niceness that I've seen before include "pleasant" , "Good nature" .)