Spit up the last of the brandy when slightly drunk (5)
I believe the answer is:
tipsy
'slightly drunk' is the definition.
(tipsy means somewhat drunk)
'spit up the last of the brandy' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'the last of' suggests the final letters.
The final letter of 'brandy' is 'y'.
'spit' back-to-front is 'tips'.
'tips'+'y'='TIPSY'
'when' acts as a link.
(Other definitions for tipsy that I've seen before include "Rather drunk" , "A bit drunk" , "1 over 8" , "affected by drink" , "Merry, squiffy" .)