It's senseless to put back the barrier (3)
I believe the answer is:
mad
'it's senseless' is the definition.
(I've seen this in another clue)
'back the barrier' is the wordplay.
'back' says the letters should be written in reverse.
'the barrier' becomes 'dam' (dam is a kind of barrier).
'dam' back-to-front is 'MAD'.
'to put' is the link.
(Other definitions for mad that I've seen before include "(Of a dog) rabid" , "Frenzied" , "Crazy" , "Berserk" , "Crackers" .)