On moving down the group of halogens, the electron gain enthalpy becomes less negative. Fluorine has less negative electron gain enthalpy than chlorine due to the smaller atomic size of the fluorine atom.
Inter electronic repulsions are stronger in the relatively small 2p orbitals of fluorine and there is no much attraction of the nucleus on the incoming electron, due to higher electron density.
Chlorine has the highest value of negative electron gain enthalpy.