Letter of zero length (7)
I believe the answer is:
omicron
'letter' is the definition.
(I know that omicron is a type of letter)
'zero length' is the wordplay.
'zero' becomes 'O' (O resembles 0).
'length' becomes 'micron' (I have seen 'Small length' mean 'micron' so perhaps 'length' could also mean 'micron').
'o'+'micron'='OMICRON'
'of' is the link.
(Other definitions for omicron that I've seen before include "A rounded character" , "Moronic (anag) - Greek letter" , "character from Athens" , "Round character" , "letter from 13 [HELLENE]" .)