Crossword Genius

Investigate dark red book (5)

Ross

I believe the answer is:

plumb

I'm a little stuck... Click here to teach me more about this clue!

'investigate' is the definition.
(I've seen this in another clue)

'dark red book' is the wordplay.
'dark red' becomes 'plum' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'book' becomes 'b' (abbreviation for book).
'plum'+'b'='PLUMB'

Can you help me to learn more?

(Other definitions for plumb that I've seen before include "Going straight up and down" , "Connect pipes" , "Exactly - perpendicular" , "Lead weight for testing depth or the perpendicular" , "Exactly - in a vertical line" .)

I've seen this clue in The Sun.
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