Blast barrier before end of construction (4)
I believe the answer is:
damn
'blast' is the definition.
(I've seen this before)
'barrier before end of construction' is the wordplay.
'barrier' becomes 'dam' (dam is a kind of barrier).
'before' is a charade indicator (letters next to each other).
'end of' indicates one should take the final letters.
The final letter of 'construction' is 'n'.
'dam'+'n'='DAMN'
(Other definitions for damn that I've seen before include "Censure" , "Condemn as worthless" , "Mild expletive" , "hang" , "Blast!" .)