HMRC device seeing dodge after unknown amount slipped into bill (3,7)

'hmrc device' is the definition.

'dodge after unknown amount slipped into bill' is the wordplay.
'dodge' becomes 'racket'.
'after' says to put letters next to each other.
'unknown amount' becomes 'x' (algebra).
'slipped into' indicates putting letters inside.
'bill' becomes 'tab' (tab is a kind of bill).
'x' going inside 'tab' is 'taxb'.
'racket' after 'taxb' is 'TAX BRACKET'.

'seeing' is the link.