You can make quick compost with grass clippings and leaves, so long as you chop and turn the mix every three days. You can also mix two parts grass clippings with every one part manure for another relatively fast compost, with no need to turn. You can also use grass clippings as mulch (lay it on 3-4 inches deep), to suppress weeds and keep moisture in soil. Or use them as a green manure. Scatter them on the soil and dig in. You could also change your mowing regime and leave the clippings on the lawn. This helps protect the lawn during long dry spells. Don’t let grass grow longer than 4cm between mowings and cut when lawn is dry.