Bird book, hardly concise (9)
I believe the answer is:
'bird' is the definition.
(brambling is a kind of bird)
'book hardly concise' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'hardly concise' becomes 'rambling' (rambling can mean verbose as opposed to concise).
(Other definitions for brambling that I've seen before include "berries" , "Type of finch" , "bird" .)