Is rising to go for a drink (5)

'for a drink' is the definition.
(Pepsi is an example)

'is rising to go' is the wordplay.
'rising' says the letters should be written in reverse (in a down clue, letters go up).
'to' says to put letters next to each other (I've seen this in other clues).
'go' becomes 'pep' (I've seen this in another clue).
'is' backwards is 'si'.
'si' after 'pep' is 'PEPSI'.