What is the significance of this quote? "His head leaned back so far that is rested against the face of a defunct mantelpiece clock."

The clock itself symbolizes the collision of the past, the present, and the future. Chapter Five is so important because Gatsby's dream is finally reaching fruition, and he is reunited with Daisy. The clock..... it's already broken; the fact that it's defunct symbolizes Gatsby's inability to either go back or move forward. of the non-dream state.