Close friend cheers up, parting eventually (8)
I believe the answer is:
intimate
'close friend' is the definition.
(intimate can be a noun: a close friend)
'cheers up parting eventually' is the wordplay.
'cheers' becomes 'ta' (both can informally mean thank you).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'parting' indicates putting letters inside (inserted letters split or part another word).
'eventually' becomes 'intime' (synonyms).
'ta' in reverse letter order is 'at'.
'at' inserted within 'intime' is 'INTIMATE'.
(Other definitions for intimate that I've seen before include "state" , "Tip off" , "cosy" , "Sharing a close and affectionate friendship" , "Hint to a familiar friend" .)