Add child movieclip such that extra part of child movieclip is hidden

hi there

i have a parent movieclip mcA and i want to add another movieclip mcB (child movieclip) inside mcA such that the extra part of mcB is hidden. The size of movieclip remains same as of mcA

For ex:-

mcA = 300x300
mcB = 400x400

then only 300x300 of mcB movieclip is visible and rest of the part is hidden.

Can somebody please tell how is this possible ???

If you have any questions please let know…

Thanks a lot in in advance