Disturb bats in allegedly empty folly (9)

'folly' is the definition.
(absurdity is a kind of folly)

'disturb bats in allegedly empty' is the wordplay.
'bats' indicates anagramming the letters (slang for mad).
'in' indicates putting letters inside.
'empty' means to remove the middle letters.
'allegedly' with its middle removed is 'ay'.
'disturb' with letters rearranged gives 'bsurdit'.
'bsurdit' placed within 'ay' is 'ABSURDITY'.