Am sure to beat at, having entered: they’re no experts (8)
I believe the answer is:
'no experts' is the definition.
The answer and definition can be both people as well as being plural nouns.
Maybe you can see an association between them that I can't see?
'am sure to beat at having entered' is the wordplay.
'to beat' indicates anagramming the letters.
'at' is a charade indicator (letters next to each other).
'having' becomes 'at' (I can't justify this - if you can you should believe this answer much more).
'entered' indicates putting letters inside.
'sure' anagrammed gives 'eurs'.
'ameurs' placed around 'at' is 'AMATEURS'.
'they're' is the link.
Can you help me to learn more?
(Other definitions for amateurs that I've seen before include "Tamer USA (anag.)" , "Non-professionals" , "inept lot?" , "sporting types" , "They're enthusiastic" .)