Ancient -- and value in it is dubious (12)
I believe the answer is:
antediluvian
'ancient' is the definition.
(antediluvian means before the biblical Flood)
'and value in it is dubious' is the wordplay.
'is dubious' indicates anagramming the letters.
'and'+'value'+'in'+'it'='andvalueinit'
'andvalueinit' with letters rearranged gives 'ANTEDILUVIAN'.
(Other definitions for antediluvian that I've seen before include "Very old-fashioned (from before the Flood?)" , "before the water's bubbled up" , "Very old (before the Flood)" , "Before the flood; ancient" .)