Nominally she's discretion itself (8)
I believe the answer is:
prudence
'discretion itself' is the definition.
I can't judge whether this definition defines the answer.
'nominally she's' is the wordplay.
I cannot quite see how this works, but
'she' could be 'pru' and 'pru' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for prudence that I've seen before include "A woman < 's > circumspection" , "Quality of being cautious and sensible, and a girl's name" , "Discretion, caution" , "Shrewdness in planning ahead" , "showing caution" .)