Almost share measure (5)
I believe the answer is:
ratio
'measure' is the definition.
(I have seen 'Proportional measure' mean 'ratio' so perhaps 'measure' could also mean 'ratio')
'almost share' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'share' becomes 'ration' (a ration is a share of a limited resource).
'ration' with its last letter removed is 'RATIO'.
(Other definitions for ratio that I've seen before include "Proportion of one thing to another" , "One-to-one, maybe" , "Relativity between two quantities" , "Relative magnitudes of two quantities" , "Relationship between two numerical quantities" .)