Comparative magnitude of bits east of desert (5)
I believe the answer is:
ratio
'comparative magnitude' is the definition.
(a ratio compares two amounts)
'bits east of desert' is the wordplay.
'bits' becomes 'io' (resembling 1 and 0, both bits in computing).
'east of' says to put letters next to each other (some letters go to the right of others).
'desert' becomes 'rat' (abandon treacherously).
'io' put after 'rat' is 'RATIO'.
'of' acts as a link.
(Other definitions for ratio that I've seen before include "Measure expressed as a proportion" , "One-to-one, maybe" , "Relation between two things, numbers say" , "eg 1:3" , "Eg, 4 to 3" .)