Reverting to drink, one's up to no good (7)

'reverting' is the definition.
('lapsing' can be a synonym of 'reverting')

'drink one's up to no good' is the wordplay.
'drink' becomes 'lap' (lapping is a kind of drinking**).
'one's' becomes 'is' (roman numeral).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'to no' becomes 'n' (common abbreviation).
'good' becomes 'g' (abbreviation).
'is' written backwards gives 'si'.
'lap'+'si'+'n'+'g'='LAPSING'

'to' acts as a link.