Crossword Genius

Girl who can cast spells? Spooner says it's dull (10)

Ross

I believe the answer is:

ditchwater

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'girl who can cast spells? spooner says it's dull' is the definition.
The definition suggests a singular noun which matches the answer.

'girl' is the wordplay.
I cannot quite see how this works, but
'girl' could be 'di' and 'di' is located in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for ditchwater that I've seen before include "What credit (anag.)" , "proverbially dull" .)

I've seen this clue in The Guardian.
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