Somewhat mediocre a.m. -- so Dad's getting a drink (5,4)
I believe the answer is:
cream soda
'a drink' is the definition.
(I've seen this before)
'somewhat mediocre a m so dad's' is the wordplay.
'somewhat' indicates a hidden word.
'CREAM SODA' is hidden in the letters of 'mediocre a m so dads'.
'getting' acts as a link.
(Other definitions for cream soda that I've seen before include "Vanilla-flavoured pop" , "Vanilla-flavoured fizzy drink" , "Carbonated soft drink" , "Sparkling stuff" .)