Putting up a barrier across river is insane (3)
I believe the answer is:
mad
'insane' is the definition.
(synonyms)
'putting up a barrier across river' is the wordplay.
'putting up' says the letters should be written backwards.
'a barrier across river' becomes 'dam'.
'dam' reversed gives 'MAD'.
'is' acts as a link.
(Other definitions for mad that I've seen before include "abandoned" , "Crackers" , "Frenzied" , "Aberrant" , "like the March Hare" .)