Fail to prevent (3)
I believe the answer is:
let
'fail to prevent' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for let that I've seen before include "Umpire's decision" , "Allow; rental period" , "Award (a contract)" , "& 15 Triple (anag.)" , "Was inclined to ignore" .)