Even older sister close to home is one paying a flat fee (6)
I believe the answer is:
lessee
'one paying a flat fee' is the definition.
(I've seen this in another clue)
'even older sister close to home' is the wordplay.
'even' indicates alternate letters (even-numbered letters are used).
'close to' indicates one should take the final letters (the 'close' to the word).
The final letter of 'home' is 'e'.
'older'+'sister'='oldersister'
The alternate letters of 'oldersister' are 'lesse'.
'lesse'+'e'='LESSEE'
'is' is the link.
(Other definitions for lessee that I've seen before include "One holds a lease" , "Leaseholder" , "I have to deal with letter" , "One granted temporary use of property" , "Contractor" .)