Am sure to beat at, having entered: they’re no experts (8)
I believe the answer is:
amateurs
'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'.
'am'+'eurs'='ameurs'
'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" .)