That is very easy. Lets suppose the name of player sprite is Zelda and name of enemy sprite is Zombie.
MOUSE on click on the zombie
SYSTEM distance (zelda.X, zelda.Y,zombie.X, zombie.Y)
Is less or equal than 100
Zombie destroy (or substract from zombie "heath")
Its always better to put imagem but on this forum it so complicated.
And istead of 100, you can put any number.