chlorine shows exceptionally higher electron gain enthalpy than that of florine.
Even if the Fluorine is most electronegative element still its electron gain enthalpy is lower than Chlorine.
In case of Fluorine the size of valence shell is so small that the upcoming electron experiences repulsions so it is not practically easy for Fluorine to gain an electron as compared to Chlorine.
On other hand in case of Chlorine the outer or valence shell is enough large to accommodate the upcoming electron. So the electron affinity of chlorine is higher.