In Series First and second number added by 4 to Get Second and Third Number and Third number is subtracted by 7 to get next number and this process will continue further.

$1+4=5$

$5+4=9$

$9-7=2$

$2+4=6$

$6+4=10$

$10-7=3$

$3+4=7$

$7+4=11$

$11-7=4$

$4+4=8$

$8+4=12$

Hence,Missing Number will be $7$.