Wager fellow joined a large deception (8)
I believe the answer is:
betrayal
'deception' is the definition.
(betraying can mean deceiving someone)
'wager fellow joined a large' is the wordplay.
'wager' becomes 'bet' (synonyms).
'fellow' becomes 'ray' (man's name).
'joined' is a charade indicator (letters next to each other).
'large' becomes 'L' (eg in clothes sizes).
'bet'+'ray'+'a'+'l'='BETRAYAL'
(Other definitions for betrayal that I've seen before include "Breach of trust" , "Treacherous act" , "Double-crossing" , "Act of treachery" , "Breaking of trust" .)