Do you have a video file you want to extract a clip from?
I found it difficult to locate this information and learned a lot in the process. Most importantly, the order of options in avconv command matters. You can refer to the manual here.
avconv -ss [start time = hh:mm:ss] -i [input filename] -t [clip duration = hh:mm:ss or in seconds] -codec copy [output filename]
For example, to create a 79 second clip from a video file that starts at 58 minutes and 25 seconds in the original video:
avconv -ss 00:58:25 -i 1995-8mm-home-video.dv -t 79 -codec copy 1995-guinea-pigs-garage.avi