Clumsy oaf left book in taxi (6)
I believe the answer is:
lubber
'clumsy oaf' is the definition.
'left book in taxi' is the wordplay.
'left' becomes 'l' (common abbreviation).
'book' becomes 'b'.
'in' is an insertion indicator.
'taxi' becomes 'uber'.
'b' inserted into 'uber' is 'ubber'.
'l'+'ubber'='LUBBER'
(Other definitions for lubber that I've seen before include "A clumsy fellow, usually with land" , "Clumsy type" , "No salt" .)