Dicky is eloquent about a book of dubious merit (12)
I believe the answer is:
questionable
'of dubious merit' is the definition.
'dicky is eloquent about a book' is the wordplay.
'dicky' indicates an anagram (dicky can mean ill or non-functioning).
'about' means one lot of letters goes inside another.
'book' becomes 'b'.
'is'+'eloquent'='iseloquent'
'iseloquent' with letters rearranged gives 'questionle'.
'a'+'b'='ab'
'questionle' enclosing 'ab' is 'QUESTIONABLE'.
(Other definitions for questionable that I've seen before include "Suspect, unproven" , "Doubtful - possibly dishonest" , "Uncertain" , "Open to doubt" , "Iffy" .)