Thus might Blanchett hold scales that mark values (9)
I believe the answer is:
calibrate
'thus might blanchett hold' is the definition.
I can't judge whether this defines the answer.
'scales that mark values' is the wordplay.
I cannot quite see how this works, but
'scales' could be 'libra' (I have seen 'The Scales' mean 'libra' so perhaps 'scales' could also mean 'libra') and 'libra' is present in the answer.
The remaining letters 'cate' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for calibrate that I've seen before include "Mark the scale of a measuring instrument" , "Mark to a standard" , "Mark positions" , "judge factors in readings" , "Mark (a gauge) with a scale of readings" .)