Speech ignoring case for proportionate measure (5)
I believe the answer is:
ratio
'proportionate measure' is the definition.
(a ratio measures a proportion)
'speech ignoring case' is the wordplay.
'speech' becomes 'oration' (oration is a kind of speech).
'ignoring case' indicates the central letters (letters which encase the word are removed).
The middle of 'oration' is 'RATIO'.
'for' is the link.
(Other definitions for ratio that I've seen before include "Eg, three to one" , "Mathematical relation of one thing to another" , "Relationship between two numerical quantities" , "figures in relationship" , "Relation between two amounts" .)