Investigate dark red book (5)
I believe the answer is:
plumb
'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" .)