The trend for ionization energy is: going from left to right on the periodic table, IE will increase, while from top to bottom, IE will decrease. There are some exceptions, however.
But they do not apply here.
Cl is right under F, they are in the same group, so IE of F is higher than that of Cl.
S and Cl are in the same period, S being to the left of Cl, so IE of Cl is higher than that of S.
Hope that makes sense.