Oh no! The brain's crazily, ridiculously complex! (5-8)
I believe the answer is:
heath robinson
'ridiculously complex' is the definition.
('Heath Robinson' can mean over-complicated)
'oh no the brain's crazily' is the wordplay.
'crazily' indicates an anagram.
'oh'+'no'+'the'+'brains'='ohnothebrains'
'ohnothebrains' with letters rearranged gives 'HEATH-ROBINSON'.
(Other definitions for heath robinson that I've seen before include "over-elaborate in style" , "Absurdly complicated in design (for a machine)" , "Ridiculously complicated in design" , "quirky illustrator" , "Cartoonist noted for fantastic machinery" .)