Furious when barrier's lifted (3)
I believe the answer is:
mad
'furious' is the definition.
(I've seen this in another clue)
'barrier's lifted' is the wordplay.
'barrier' becomes 'dam' (dam is a kind of barrier).
'lifted' says the letters should be written in reverse (going up in down clue).
'dam' reversed gives 'MAD'.
'when' acts as a link.
(Other definitions for mad that I've seen before include "abandoned" , "Deranged" , "(Of a dog) rabid" , "Angry or insane" , "Crazy" .)