The correct procedure to allow you to checkout that file would be to include you in the list of users exempted on that lock: see cleartool lock
man page.
But you need to be the owner of the pvob in which the activity or the current Stream has been declared, or root (or privileged user on Windows).
If the activity only is locked, another way would be for you to create another activity.
cleartool mkact anotherActivity
and try your checkout then. It should have unset the current (locked) activity, and set a new one.
But if the Stream itself is locked (or even the vob/pvob), then you need to ask your ClearCase admin for an unlock.
See for instance IBM technote "Lock on activity prevents operation change activity", which illustrate how that "activity locked" error message is, in this case, misleading.
But that was for CC2003, and I suppose you are using a more recent version of ClearCase.