Bats best in a helmet (7)
I believe the answer is:
basinet
'helmet' is the definition.
(I know that basinet is a type of helmet)
'bats best in a' is the wordplay.
'bats' is an anagram indicator (slang for mad).
'best'+'in'+'a'='bestina'
'bestina' anagrammed gives 'BASINET'.
(Other definitions for basinet that I've seen before include "Medieval helmet with visor" , "Light steel helmet" , "old-fashioned headgear" .)