Starts using very easy affixer on part of the eyeball (4)
I believe the answer is:
uvea
'part of the eyeball' is the definition.
(I've seen this in another clue)
'starts using very easy affixer' is the wordplay.
'starts' suggests taking the first letters.
The first letters of 'using very easy affixer' is 'UVEA'.
'on' is the link.
(Other definitions for uvea that I've seen before include "bit of mince pie!" , "The pigmented layer of the eye" , "Pigmented part of eye" , "Iris's part" , "bit of a looker" .)