It's crazy putting barrier up (3)
I believe the answer is:
mad
'it's crazy' is the definition.
(synonyms)
'barrier up' is the wordplay.
'barrier' becomes 'dam' (dam is a kind of barrier).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'dam' reversed gives 'MAD'.
'putting' is the link.
(Other definitions for mad that I've seen before include "Extremely foolish" , "Roused to anger" , "Crazy" , "Frenzied" , "Frantic" .)