Clean or Flaps
Extending Flap does a number of things. The camber of the wing is increased and thus more lift is produced for a given airspeed. Adversely, more drag is produced also. The aeroplane will have a better Lift/Drag ratio with the flaps retracted and when flown at the best glide speed will maximise the glide distance. Flying faster or slower than Best L/D Speed will reduce the glide distance. Flaps are best left up until a landing is assured.