Greek alphabet letter (6)
(Other definitions for rho that I've seen before include "Greek character" , "Gr. letter written as P" , "Greek letter R" , "letter to Athens" , "Something like 25 [ETA]" .)
(Other definitions for eta that I've seen before include "When one is due" , "Greek letter - time due (abbr)" , "when you can hope to receive it" , "Letter from abroad" , "When's end of journey predicted" .)
(Other definitions for tau that I've seen before include "letter from foreigner" , "19th letter of the Greek alphabet" , "Greek letter; cross" , "Foreign character" , "Homeric character" .)
(Other definitions for lambda that I've seen before include "Greek character" , "Teasing entertainer" , "character from Greece" , "Eleventh Greek letter" , "part of the skull" .)
'greek alphabet letter' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for omicron that I've seen before include "A rounded character" , "Greek short 'o'" , "character in Europe" , "old letter" , "letter from 13 [HELLENE]" .)