First you want to turn off snapping so that you can trim and not snap to the grid that you are using. Then you hover the mouse over the front edge of the clip and you should get a little icon thing then you can click and hold then drag and drop the beginning to where you need it.
Can I delete reaper peaks file?
You can actually delete these peak files whenever you want, REAPER will just create them again as needed and it only takes a few seconds for most projects. ... So what about all the old peak files littered all over your hard drive? You can delete them.
What are peaks in Reaper?
the peak files are just temporary files that represent the waveform graphics used in the track. it's not converting anything. you can delete the reapeak files and they'll be recreated as needed. if they bug you, you can set reaper to put them all in a folder somewhere else.