Capital cheers up birds (6)
I believe the answer is:
athens
'capital' is the definition.
(capital of Greece)
'cheers up birds' is the wordplay.
'cheers' becomes 'ta' (both can mean thank you).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'birds' becomes 'hens' (hen is a type of bird).
'ta' back-to-front is 'at'.
'at'+'hens'='ATHENS'
(Other definitions for athens that I've seen before include "City of the Parthenon" , "Capital of Greece" , "city in Greece" , "Hasten to European city" , "6 capital" .)