Boat built to withstand a great flood (3)
I believe the answer is:
ark
'boat built to withstand a great flood' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ark that I've seen before include "Biblical vessel" , "Noah's craft" , "Great biblical boat" , "Rescue boat" , "Noah's vessel - in which the Torah scrolls are kept" .)