Island postman regularly delivering letter (4)
I believe the answer is:
iota
'letter' is the definition.
(iota is a kind of letter)
'island postman regularly' is the wordplay.
'island' becomes 'i'.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternate letters of 'postman' are 'ota'.
'i'+'ota'='IOTA'
'delivering' is the link.
(Other definitions for iota that I've seen before include "Just a jot, like Greek letter" , "Just a jot, from Greek alphabet" , "letter from Greece" , "Greek letter; tiny amount" , "Greek letter - minimal amount" .)