way1:
awk -F',' '$2 ~ /a/ { print }' input
way2:
grep -e ".a" < inpuy

sample of input file:
a,b,c,d
1,4a337485,2,54
4,2a4645647,4,56
6,5a3489556,3,22
9,,3,12
10,0,34,45
24,4a83944,3,22
45,,435,34

expected output file:
a,b,c,d
1,4a337485,2,54
4,2a4645647,4,56
6,5a3489556,3,22
24,4a83944,3,22