Letter after theta (4)
I believe the answer is:
iota
'letter after theta' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for iota that I've seen before include "Greek letter; tiny amount" , "Ninth letter of Greek alphabet, small amount" , "Gr. letter; small amount" , "letter from Greece" , "Tiniest scrap" .)