Riot Act detains this Greek character (4)
I believe the answer is:
iota
'greek character' is the definition.
(letter from the Greek alphabet)
'riot act detains' is the wordplay.
'detains' says the answer is hidden in the clue (the answer is detained by the clue).
'IOTA' is hidden within 'riot act'.
'this' is the link.
(Other definitions for iota that I've seen before include "(Not one) little bit" , "Gr. letter; small amount" , "'Very small amount, jot (4)'" , "Hint" , "Contrivance" .)