Barrier overturned? Crazy! (3)
I believe the answer is:
mad
'crazy' is the definition.
(synonyms)
'barrier overturned?' is the wordplay.
'barrier' becomes 'dam' (dam is a kind of barrier).
'overturned?' says the letters should be written backwards.
'dam' back-to-front is 'MAD'.
(Other definitions for mad that I've seen before include "Deranged" , "Berserk" , "abandoned" , "Insane - furious" , "Crackers" .)