Reassignment of structure values

Hello! Having trouble with structure arrays! I have a large (150Mb) structure of the format ADV.Day.M.Pos.Depth All values are numerical. I need to remove certain values and have identified the ones i don't want in as an index. All i want to do is crop these values so have written a for loop in the vain of

G=ADV.Day.M.Pos.Depth
G (index)=[]

But now i want to replace the original data but it keeps coming up with ' Expected one output from a curly brace or dot indexing expression but there were 9 results". Could you please help me out with the syntax? I've been playing with structfun and num2cell but to no avail...

[Jan]

Please post your code.