Q: "Why shouldn't you tell secrets on a farm?" A: "Because the corn has ___!" (4)
I believe the answer is:
ears
(Other definitions for ears that I've seen before include "Spikes of corn" , "Corn; organs" , "''He who has ... to hear, let him hear'' (bible)" , "'''Friends, Romans, countrymen, lend me your ...'' (Julius Caesar) (4)'" , "One's hearing aids" .)