Tricksters in gambling game welcoming posh groups (11)

'tricksters' is the definition.
(mountebank is a kind of trickster)

'gambling game welcoming posh groups' is the wordplay.
'gambling game' becomes 'monte' (Three-card Monte is a gambling game).
'welcoming' is an insertion indicator (in sense of 'inviting in').
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'groups' becomes 'banks' (bank is a kind of group).
'monte' enclosing 'u' is 'mounte'.
'mounte'+'banks'='MOUNTEBANKS'

'in' acts as a link.