create single-value measure from a measure array

I need some education on MDX.. :) I have Measure A which gets calculated across Dimensions Dim X, DimY and DimMonthly.

is there a way or maybe function to create Measure B derived from measure A but selects only single Measure A value attributed to  LastMonthValue (e.g. relative to today, Nov 2013 for today) and crosses DimMonthly on Nov 2013?

What should I use

filter - Filter Nov 2013?

Tail function with item(0)?

or create  Measure B based on default named set = Nov 2013

or it could be done only using SELECT?

...?