I study tip about folly (9)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
stupidity
'folly' is the definition.
('stupidity' can be a synonym of 'folly')
'i study tip about' is the wordplay.
'about' indicates an anagram.
'i'+'study'+'tip'='istudytip'
'istudytip' anagrammed gives 'STUPIDITY'.
(Other definitions for stupidity that I've seen before include "Folly" , "Lack of intelligence" , "Dull-wittedness" .)