Measure by figure to block alien (6)
I believe the answer is:
extent
'measure' is the definition.
(I've seen this before)
'by figure to block alien' is the wordplay.
I cannot quite understand how this works, but
'by' could be 'x' (multiplication sign) and 'x' is found within the answer.
'figure' could be 'ten' (ten is a number or figure) and 'ten' is located in the answer.
'alien' could be 'et' (extra terrestrial) and 'et' is located in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for extent that I've seen before include "Area; size" , "Range or scope of something" , "reach?" , "Degree - amount - scope" , "stretch" .)