Drink behind old man's decline (4,2)
I believe the answer is:
pass up
'decline' is the definition.
(I know that decline can be written as pass up)
'drink behind old man's' is the wordplay.
'drink' becomes 'sup' (to sup is to drink).
'behind' says to put letters next to each other.
'old man's' becomes 'pa's' (both can mean someone's father).
'sup' after 'pas' is 'PASS UP'.
(Other definitions for pass up that I've seen before include "Turn (something) down" , "Miss" , "Fail to take advantage of" , "Refrain from seizing (an opportunity)" , "Showing neglect" .)