Drifts back, flushed, into “The Flying Swan” (7)

'drifts' is the definition.
('wander' can be a synonym of 'drift')

'back flushed into the flying swan' is the wordplay.
'back' says the letters should be written backwards.
'flushed' becomes 'red' ('red' can be a synonym of 'flushed').
'into' means one lot of letters goes inside another.
'the flying' indicates anagramming the letters.
'red' written backwards gives 'der'.
'swan' with letters rearranged gives 'wans'.
'der' placed inside 'wans' is 'WANDERS'.