Frenzied expression as English promise to pay up (4)

'frenzied expression' is the definition.
(exclamation made in during Bacchic revels)

'english promise to pay up' is the wordplay.
'english' becomes 'E' (abbreviation).
'promise to pay' becomes 'IOU' (I owe you, a promise to pay a debt).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'iou' reversed gives 'uoi'.
'e'+'uoi'='EUOI'

'as' is the link.