Sensible as Doreen is silly? (8)
I believe the answer is:
reasoned
'sensible' is the definition.
(thesaurus)
'as doreen is silly?' is the wordplay.
'is silly?' indicates anagramming the letters.
'as'+'doreen'='asdoreen'
'asdoreen' is an anagram of 'REASONED'.
(Other definitions for reasoned that I've seen before include "Argued, concluded" , "Worked out through logic and sense" , "Logically derived" , "Dear ones (anag)" , "Spoke logically, thought and drew conclusions" .)