Drink pep with beer (6,3)
I believe the answer is:
ginger ale
'drink' is the definition.
(ginger ale is a kind of drink)
'pep with beer' is the wordplay.
'pep' becomes 'ginger' ('ginger' can be a synonym of 'pep').
'with' means one lot of letters go next to another.
'beer' becomes 'ale' (I've seen this before).
'ginger'+'ale'='GINGER ALE'
(Other definitions for ginger ale that I've seen before include "Fizzy drink" , "Spicy, sweetened non-alcoholic drink" , "It won't make you drunk!" , "One drink" , "pop" .)