Hello,
I presume that you have created LUN under qtree ( for using snapvault)
Your snapvaulted volume is read only, so when you create a clone of this volume, the clone will be read only.
You have to run  "snapmirror convert" and then a "snapmirror break" to make the clone  writebale ( need a snapmirror license). 
For example : snapvaulted volume is volume1 and qtree qtree1
- You create a snapshot for volume1 on destination filer : daily.0
- you create a clone of volume1 based on snapshot daily.0
     vol clone create CL_volume1 -s none -b volume1 daily.0
- you disable snapmirror and snapvault
     snapmirror off
     options snapvault.enable off
-  you switch to diag level
     priv set diag
- snapmirror convert
          snapmirror convert /vol/CL_volume1/qtree1
- you active snapmirror and snapvault
         snapmirror on
          options snapvault.enable on
- you break snapmirror 
          snapmirror quiesce /vol/CL_volume1/qtree1
          snapmirror break  destination:/vol/CL_volume1/qtree1
 
I have done this for snapvaulted qtree, I think that it will be also OK for LUN under qtree.
 
I hope that will help.
 
Said