Very good and not affected by: it's miraculous (12)
I believe the answer is:
supernatural
'it's miraculous' is the definition.
('supernatural' can be similar in meaning to 'miraculous')
'very good and not affected' is the wordplay.
'very good' becomes 'super' (I've seen this before).
'and' says to put letters next to each other.
'not affected' becomes 'natural' ('natural' is the opposite to 'affected').
'super'+'natural'='SUPERNATURAL'
'by' acts as a link.
(Other definitions for supernatural that I've seen before include "Mysterious" , "Magic" , "Miraculous, magical" , "Not of this world" , "outside the known law?" .)