Cheers echoed for so long (2-2)
I believe the answer is:
ta ta
'so long' is the definition.
(both are ways of saying goodbye)
'cheers echoed' is the subsidiary indication.
'cheers' becomes 'ta' (I've seen this in other clues).
'echoed' says to double the letters (I've seen 'echo of' mean this).
'ta' duplicated is 'tata'.
'for' is the link.
(Other definitions for ta ta that I've seen before include "Remark of farewell or thanks" , "Bye-bye" , "Goodbye (colloq.)" , "So long!" , "Informal remark in farewell or gratitude" .)