Relationship great tailor regularly provides (5)
I believe the answer is:
ratio
'relationship' is the definition.
(relationship between two numbers)
'great tailor regularly provides' is the wordplay.
'regularly provides' means one should take alternating letters.
'great'+'tailor'='greattailor'
The alternate letters of 'greattailor' are 'RATIO'.
(Other definitions for ratio that I've seen before include "Relation between two amounts" , "Proportional measure" , "Proporation, quantitative relation" , "Relationship found in a riot" , "Quotient of two numbers" .)