The electron affinity of an element is the energy given off when a neutral atom in the gas phase gains an extra electron to form a negatively charged ion. A fluorine atom in the gas phase, for example, gives off energy when it gains an electron to form a fluoride ion.
F(g) + e- -----> F-(g) deltaHo = -328.0 kJ/mol
The electron cloud of F is more dense than that of Cl because Cl is larger in size. So electrons can't enter in Florine's valence shell because of electrostatic force of rapultion of electro &electron cloud. So, electron affinity of fluorine is comparatively less than chlorine.