This Greek character is heard in sequence (3)
I believe the answer is:
rho
'this greek character' is the definition.
(letter from the Greek alphabet)
'heard in sequence' is the wordplay.
'heard in' indicates a 'sounds like' (homophone) clue.
'sequence' becomes 'row' (row is a kind of sequence).
'row' is a homophone of 'RHO'.
'is' acts as a link.
(Other definitions for rho that I've seen before include "Greek letter R" , "17th Greek letter" , "Greek P" , "letter to Athens" , "Greek letter used as symbol for density" .)