Young slender person? Drink up! (4)
I believe the answer is:
'young slender person?' is the definition.
(slip can mean a slender person)
'drink up' is the wordplay.
'drink' becomes 'pils' (type of beer).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'pils' written backwards gives 'SLIP'.
(Other definitions for slip that I've seen before include "Mistake -- small piece of paper" , "release > voucher" , "failure" , "shift" , "Mistake; garment" .)