Spoil yours truly with second drink (4,2)
I believe the answer is:
mess up
'spoil' is the definition.
(I know that spoil can be written as mess up)
'yours truly with second drink' is the wordplay.
'yours truly' becomes 'me' (the person that set this crossword).
'with' says to put letters next to each other.
'second' becomes 's' (common abbreviation).
'drink' becomes 'sup' (to sup is to drink).
'me'+'s'+'sup'='MESS-UP'
(Other definitions for mess up that I've seen before include "do psychological harm" , "Fluff" , "Bungle" , "Botch" , "Spoil" .)