Singer adopting a retro sound, once overlooked - bless (7)
I believe the answer is:
beatify
'bless' is the definition.
(thesaurus)
'singer adopting a retro sound' is the wordplay.
'singer' becomes 'bey' (I can't justify this - if you can you should believe this answer much more).
'adopting' indicates putting letters inside (inserted letters are taken on or adopted).
'retro' shows that the letters should be reversed in order (retro is Latin for 'backwards').
'sound' becomes 'fit' (associated in meaning).
'fit' in reverse letter order is 'tif'.
'a'+'tif'='atif'
'bey' placed around 'atif' is 'BEATIFY'.
'once overlooked' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for beatify that I've seen before include "Make blessed" , "Proclaim praiseworthy" , "Exalt" .)