Poet's black book one that's devious keeps (4)
I believe the answer is:
ebon
'poet's black' is the definition.
(I've seen this in another clue)
'book one that's devious keeps' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'that's devious' indicates anagramming the letters.
'keeps' is an insertion indicator (inserted letters are kept inside).
'one' with letters rearranged gives 'eon'.
'b' inserted within 'eon' is 'EBON'.
(Other definitions for ebon that I've seen before include "Poetic dark" , "Dark in colour" , "Deep black" .)