Queen singer holding one more flush (6)
I believe the answer is:
richer
'more flush' is the definition.
('rich' can be similar in meaning to 'flush')
'queen singer holding one' is the wordplay.
'queen' becomes 'r' (abbreviation for regina, e.g. in ER ).
'singer' becomes 'cher' (American singer).
'holding' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'r'+'cher'='rcher'
'rcher' going around 'i' is 'RICHER'.
(Other definitions for richer that I've seen before include "More wealthy" , "Wealthier" , "With more money" , "More affluent" , "Having more wealth" .)