Proportion great tailor regularly achieves (5)
I believe the answer is:
ratio
'proportion' is the definition.
(I know that proportion can be written as ratio)
'great tailor regularly achieves' is the wordplay.
'regularly achieves' indicates alternate letters (letters taken at regular intervals).
'great'+'tailor'='greattailor'
The alternating letters of 'greattailor' are 'RATIO'.
(Other definitions for ratio that I've seen before include "Proportion, relationship" , "Relative magnitude" , "9 to 5, say?" , "figures in relationship" , "Eg, 4 to 3" .)