Gullible senate shifts from the centre (4)
I believe the answer is:
naif
'gullible' is the definition.
(similar in meaning)
I cannot understand how the rest of the clue works.
(Other definitions for naif that I've seen before include "Unworldly" , "somewhat ingenuous" , "Inexperienced" , "unaffected simplicity" , "Unsophisticated" .)