Folly for baseball teams to plug latrines (9)
I believe the answer is:
looniness
'folly' is the definition.
(I have seen 'Such folly' mean 'looniness' so perhaps 'folly' could also mean 'looniness')
'baseball teams to plug latrines' is the wordplay.
'baseball teams' becomes 'nines' ('nine' can be a synonym of 'baseball team').
'to plug' is an insertion indicator (to plug can mean to block a gap within something).
'latrines' becomes 'loos'.
'nines' put into 'loos' is 'LOONINESS'.
'for' acts as a link.
(Other definitions for looniness that I've seen before include "Insanity" , "Such folly" .)