River claims posh Charlie Dickens (5)
I believe the answer is:
deuce
'dickens' is the definition.
(synonyms)
'river claims posh charlie' is the wordplay.
'river' becomes 'dee' (River Dee).
'claims' indicates putting letters inside.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'charlie' becomes 'C' (phonetic alphabet: alpha, bravo, charlie etc.).
'u'+'c'='uc'
'dee' placed around 'uc' is 'DEUCE'.
(Other definitions for deuce that I've seen before include "score at Wimbledon?" , "with equal score" , "Dickens" , "Two in cards - score in tennis" , "More than one" .)