The electron gain enthalpies of halogen are as given below: $\mathrm{F}=-332,\mathrm{Cl}=-349,\mathrm{Br}=-324,\mathrm{I}=-295\mathrm{kJ}{\mathrm{mol}}^{-1}$

The less negative for F as compared to that of $\mathrm{Cl}$ is due to: 

Due to the short size of the F atom, the tight 2p subshell's strong electron-electron repulsions prevent the incoming electron from being taken as readily as it is with Cl (less electron-electron repulsions).
Because of this, chlorine has a greater negative electron gain enthalpy than fluorine.