Two six-footers importing regular doses of insulin for payee (9)
I believe the answer is:
annuitant
'payee' is the definition.
(person paid an annuity)
'two six-footers importing regular doses of insulin' is the wordplay.
'two six-footers' can mean two substitutions for 'six-footer'.
'six-footer' becomes 'ant' (I've seen this before).
'six-footer' becomes 'ant' (I've seen this before).
'importing' means one lot of letters goes inside another.
'regular doses of' means one should take alternating letters (letters taken at regular intervals).
The alternate letters of 'insulin' are 'nui'.
'ant'+'ant'='antant'
'antant' going around 'nui' is 'ANNUITANT'.
'for' is the link.
(Other definitions for annuitant that I've seen before include "One receiving payment" , "one receiving regular payments" , "One with a regular income from a lump sum" .)