I study tip about folly (9)
I believe the answer is:
Here is my best explanation:
'folly' is the definition.
('stupidity' can be a synonym of 'folly')
'i study tip about' is the subsidiary indication.
'about' is an anagram indicator.
'istudytip' with letters rearranged gives 'stupidity'.
(Other definitions for stupidity that I've seen before include "Folly", "Dull-wittedness", "Lack of intelligence" and "lack".)