Lose old bet newly regarded as invalid (8)
I believe the answer is:
obsolete
'invalid' is the definition.
Both the answer and definition are adjectives. Maybe there's a link between them I don't understand?
'lose old bet newly' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'newly' indicates anagramming the letters.
'lose'+'o'+'bet'='loseobet'
'loseobet' anagrammed gives 'OBSOLETE'.
'regarded as' is the link.
Can you help me to learn more?
(Other definitions for obsolete that I've seen before include "Old hat" , "being out of practice" , "Antiquated" , "No longer useful" , "Out-of-date" .)